Update jobscripts (fixes #258)

Updated the jobscripts using Midas' updated jobscript as example. The jobscript-hetjob.sh is the same as the Midas script, with a few modifications to fit climix. The main difference is the argument list to srun at the bottom of the script. Note: the --cpus-per-task setting is now 2 instead of 4, same as in Midas.

I had to add client shutdown in the __exit__ method of the ExternalScheduler class (copied code from Midas), in order for the nodes to be released when jobscript-hetjob.sh is finished.

I haven't changed the file names.

jobscript-srun.sh is deleted.