Schedule pure Python jobs

[nilsvu]

We want to be able to do spectre run MY_SCRIPT.py to run a Python script in a job on a cluster. Currently this only works with C++ executables. Running a Py script would help so we can run multiple executables in the same job, so we don't have to go through the queue e.g. for each iteration of the initial data control loop. Currently this can be worked around with the Next section in the input file, but that's a bit complicated.