Add tutorial on git workflow (including figures)

tutorials/0_Git_and_GitHub_workflow.md

+Using git and GitHub to manage your REMIND code
+================
+David Klein (<dklein@pik-potsdam.de>), Kristine Karstens (<karstens@pik-potsdam.de>)
+15 February, 2020
+
+# 1 Introduction
+
+REMIND and related R packages are published as open source software on GitHub. 
+There are various workflows, how to use GitHub for collaborative
+software development. We want to give some general remarks on our
+prefered one. Note the following:
+
+  - **Git** is a free and open source distributed version control system
+    designed to handle everything from small to very large projects with
+    speed and efficiency (from: <https://git-scm.com/> - very good help,
+    documentation, tutorial page for git).
+
+  - **GitHub** is an US American company (owned by Microsoft) that provides
+    hosting for software development version control using Git. (GitLab,
+    SourceForge and various more are open-source alternatives to
+    GitHub.)
+    
+
+### Learning objectives
+
+The goal of this exercise is to set up REMIND for collaborative working.
+After completion of this exercise, you'll be able to:
+
+1.  Fork and clone your personal REMIND repository
+2.  Keep your personal repository upto date with the REMIND main repository
+3.  Understand the basic workflow including pull requests and branches.
+4.  Have heard some very basic git commands and know where to find more help.
+    
+# 2 GitHub workflow
+
+## 2.1 Fork (on the GitHub server)
+
+<img src="figures/git-1-setup.png" width="100%" style="display: block; margin: auto;" />
+
+Every code development (even bugfixes) will be merged into our main
+repository under <https://github.com/remindmodel/remind> with the help
+of so called pull requests. Pull requests give us control over the
+changes entering our branches. To create a pull request, we use personal
+or institutional forks. They have to be kept up-to-date with our
+upstream repository (the original remindmodel fork).
+
+> **Exercise**: Visit us on <https://github.com/remindmodel/remind> and
+> create your own fork by clicking at 'fork' (at the upper right).
+
+## 2.2 Clone (download your personal repository)
+
+It is basically possible to change the code just using the GitHub
+interface, but since you want to test and run your code locally you have
+to clone the repository from your fork anyway. This can be done using
+the 'https' or 'ssh' adresse of your fork together with the `git clone`
+command (cmd/bash/GUI) at your machine. We recommand to upload an
+ssh-key and use ssh to connect to GitHub.
+
+> **Exercise**: Visit your fork and clone the repository at your
+> machine.
+
+## 2.3 Push (to your personal repository)
+
+<img src="figures/git-2-pushing.png" width="100%" style="display: block; margin: auto;" />
+
+When you start making your first changes to the code at your local copy,
+we strongly recommend to do a tutorial to get familiar with the basic
+commands in git. We have compiled a typical workflow at the end of this tutorial (section 3) (you can also have a look on 'git cheat sheets' like
+<https://about.gitlab.com/images/press/git-cheat-sheet.pdf>).
+
+After you have added your changes locally, push (upload) them to your personal remote repository.
+
+## 2.4 Pull (updates from the main repository)
+
+To keep your fork up-to-date with the upstream repository you can use
+the GitHub interface. Via `Compare` you are able to check, if the
+upstream is some commits ahead. If so merge these new changes into
+your fork. 
+
+<img src="figures/git-3-add-main.png" width="100%" style="display: block; margin: auto;" />
+<img src="figures/git-4-pull-main.png" width="100%" style="display: block; margin: auto;" />
+
+You can also do this merging procedure in your local copy by adding 
+both your fork and the upstream repository as so called 'remotes' to 
+your local repository.
+
+> **Exercise**: Check, if there is anything to merge from the upstream
+> repository into your fork. If so, merge it into the local clone of your 
+> personal fork.
+
+## 2.5 Pull request
+
+After you have commited your changes locally, merged the latest updates from the remote main repository and 
+pushed the result to your remote personal repository it is time to get it integrated
+into the remote main repository. Insted of pushing them directly into the remote main repository we use so called
+pull requests. Pull requests are proposed changes to a repository submitted by you and accepted or rejected by a 
+repository's collaborators. Like issues, pull requests each have their own discussion forum.
+
+<img src="figures/git-5-pull-request.png" width="100%" style="display: block; margin: auto;" />
+<img src="figures/git-7-pull-request-github-1.png" width="100%" style="display: block; margin: auto;" />
+<img src="figures/git-8-pull-request-github-2.png" width="100%" style="display: block; margin: auto;" />
+
+## 2.6 All steps in one figure
+
+<img src="figures/git-6-all-in-one.png" width="100%" style="display: block; margin: auto;" />
+<img src="figures/git-11-rules.png" width="100%" style="display: block; margin: auto;" />
+
+# 3 Incorporate your code changes by using branches
+
+<img src="figures/git-9-branching.png" width="100%" style="display: block; margin: auto;" />
+<img src="figures/git-10-branching-advanced.png" width="100%" style="display: block; margin: auto;" />