Feature: Cleanup depcode.py and related files

[yardasol]

Background and motivation

I read a lot of code while working on OpenMC all summer and think we can clean up depcode.py and related files.

Description of idea

• Refactor the current depcode.py into three different files:
  • abc.py : holds abstract base classses like Depcode
  • openmc_depcode.py : holds OpenMCDepcode class
  • serpent_depcode.py : holds SerpentDepcode class
• Cleanup variable names, import statements, and docstrings

Implementation details

This should be fairly straightforward.

Potential snags

This could take longer than I think it might.