add testOneRegi option to refactored start scripts

• testOneRegi runs are useful for fast development iterations ("do these changes break the model, or does it still run"), so setting them up should be fast as well

• supplying the parameter --testOneRegi to start.R will ignore any scenario_config file provided and modify the default configuration to start a testOneRegi run in the directory output/testOneRegi with no output processing

• the old oneRegi.R script and oneRegi.cfg configuration are deleted

• existing output/testOneRegi directories are overwritten

@dklein-pik Sorry about the white space changes. That's not me, but RStudio. You can hide them from the options (gear icon) in the "Files changed" view. Or append ?w=1 to the URL.