clean up planckton and add linting checks to CI

[jennyfothergill]

fixes #25

Added:

• Code has been refactored:
  • all typed compounds have gaff added in their name so it is more clear what FF they are typed with (e.g. P3HT_typed.mol2 --> P3HT-gaff_typed.mol2)
  • force_fields directory renamed to forcefields
  • COMPOUND_FILE and FORCE_FIELD dicts renamed to COMPOUND and FORCEFIELD
  • deleted cml and untyped mol2 files in compounds directory
• Added pre-commit yaml to automatically lint the code:
  • many whitespace changes were made (eol and eof whitespace deleted)
  • docstrings updated

Need to add:

• [x] dev instructions for how to set up precommit
• [x] style guide -- fixes #10

[codecov[bot]]

Codecov Report

Merging #42 (9da6fc4) into master (8dd4ee6) will increase coverage by 0.09%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   98.32%   98.42%   +0.09%     
==========================================
  Files           5        6       +1     
  Lines         179      190      +11     
==========================================
+ Hits          176      187      +11     
  Misses          3        3              

Impacted Files	Coverage Δ
planckton/utils/solvate.py	100.00% <ø> (ø)
planckton/__init__.py	100.00% <100.00%> (ø)
planckton/forcefields/__init__.py	100.00% <100.00%> (ø)
planckton/init.py	100.00% <100.00%> (ø)
planckton/sim.py	96.05% <100.00%> (+0.10%)	:arrow_up:
planckton/utils/units.py	100.00% <100.00%> (ø)

[jennyfothergill]

I think I should switch this to https://pre-commit.com/ and https://github.com/pre-commit/action