Modified the isisVarInit.py script (#3945)

* Removed version number from filename.

* feat: Changed the name of the VarInit script, changed how it worked
internally, and made it write out isis-activate.* and isis_deactivate.*
files.

* Fix: Now has correct csh syntax, updated mkdir() with a proper docstring
and to return a string (only main should print(), functions shouldn't or
should log, but that's overkill for this).