Update apply_forcefield and molecule forcefield

[marjanalbooyeh]

This PR applies following changes to the Molecule and System classes:

1. Move force_field and auto_scale parameters from System init to apply_forcefield method.
   Since applying forcefield is now done after system initialization, it makes sense to move all forcefield related parameters to apply_forcefield.
   The apply_forcefield method always sets the reference_values dictionary. When auto_scale=True the reference values will be set based on the maximum parameter value and when its false, the values are set to 1 with default gmso units. As a result, we should always use reference_values as the base_units parameter when calling gmso hoomd functions.

2. Added base classes for forcefield
   Two new classes are added to the base module: BaseXMLForcefield and BaseHOOMDForcefield. All the forcefield classes defined in the library now inherit from one of these two base classes. The base classes do not have any additional functionalities, they are defined to ensure all subclasses have similar attribute names. This consistency in attribute names is needed in forcefield validations done in System and Molecule classes.

3. Allow forcefield to be specified in Molecule class
   We can pass a valid forcefield to the Molecule class when initializing the molecules. In this case, apply_forcefield can be called without passing the forcefields and the system class uses the forcefields from the molecules.
   Note: Previously, the Molecule class had the forcefield parameter but the accepted types were either direct xml files or list of hoomd objects. With the changes in this PR, only valid forcefields, i.e. subclasses of the base forcefield classes, can be passed to the Molecule.

[codecov]

Codecov Report

Merging #62 (a0da905) into main (d71f21e) will increase coverage by 0.06%.
The diff coverage is 96.20%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   93.63%   93.70%   +0.06%     
==========================================
  Files          18       19       +1     
  Lines        1383     1397      +14     
==========================================
+ Hits         1295     1309      +14     
  Misses         88       88              

Files	Coverage Δ
hoomd_organics/base/__init__.py	100.00% <100.00%> (ø)
hoomd_organics/base/forcefield.py	100.00% <100.00%> (ø)
hoomd_organics/base/molecule.py	96.55% <100.00%> (-0.08%)	⬇️
hoomd_organics/library/__init__.py	100.00% <ø> (ø)
hoomd_organics/library/forcefields.py	100.00% <100.00%> (ø)
hoomd_organics/utils/__init__.py	100.00% <100.00%> (ø)
hoomd_organics/utils/base_types.py	100.00% <ø> (ø)
hoomd_organics/utils/ff_utils.py	89.58% <100.00%> (-2.34%)	⬇️
hoomd_organics/base/system.py	90.61% <92.50%> (+1.11%)	⬆️

... and 1 file with indirect coverage changes

[chrisjonesBSU]

LGTM, Thanks!! I just left one comment about a doc string.