update start.R with env shebang and set executable flag

Michaja Pehlrequested to merge
github/fork/0UmfHxcvx5J7JoaOhFSs5mncnisTJJ6q/dev__Rscript_env into develop
  • makes it executable from the command line without invoking Rscript explicitly
  • saves on typing, makes for more relaxed REMIND work ;)
  • could be applied to other .R scripts as well

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