Refactor apply_forcefield

[chrisjonesBSU]

This PR is related to #55 and a comment in there. This opens up the _apply_forcefield and rather than it being called automatically inside of System.__init__ is has to be called manually. The forcefield related parameters are moved out of System.__init__ and into apply_forcefield and the PPPM arguments are added.

@marjanAlbouye and I have gone back and forth between thinking about how the appy_forcefield method should be handled, so let me know what you think and if you recognize any issues that might pop up from these changes.

[codecov]

Codecov Report

Merging #61 (637efa1) into main (ff3198e) will decrease coverage by 0.06%.
The diff coverage is 95.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
- Coverage   93.69%   93.63%   -0.06%     
==========================================
  Files          18       18              
  Lines        1379     1382       +3     
==========================================
+ Hits         1292     1294       +2     
- Misses         87       88       +1     

Files	Coverage Δ
hoomd_organics/base/system.py	89.49% <95.45%> (-0.27%)	⬇️

[marjanalbooyeh]

LGTM! Thank you