Documentation for core/energy_pdf.py

[JannisNe]

Is your feature request related to a problem? Please describe.
I'm always frustrated when there is no documentation for code.

Describe the solution you'd like
I want documented code. More so probably people from outside using Flarestack.

Describe alternatives you've considered
Not adding documentation. Makes it harder for everyone though.

Additional context
This is necessary if a JOSS paper is the goal.

[JannisNe]

@robertdstein Is the EnergyPDFConstructor still being used or an inactive relic?

[robertdstein]

So I just checked, and I believe it is a relic. If the code is not being used/imported anywhere it is probably redundant. The commented code below is also a (python2) relic that can be removed.