Improve split_on_off functions

The results now contain a column to which off region they were selected and off keys are renamed
to their on equivalents.

So for example a result might look like

>>> on, off = split_on_off_source_independet(...)
>>> print(off.head())
off_region Theta 
1               0.2
3               0.5

So first event is from off region 1 and has Theta_Off_1 = 0.2