[WIP] Bond parameter interpolation based on Wiberg bond orders

[trevorgokey]

• Update docstrings/documentation, if applicable

• Update changelog

• Add a test force field with a <Bonds version="0.3" potential="harmonic" fractional_bondorder_method="Wiberg" fractional_bondorder_interpolation="linear"> element

• Add functionality to use user-specified WBO instead of default (e.g. AM1)

• Fractional bond order getter from molecule, and/or individual bonds? Already present for Bond objects. Data type return for molecule level getter?

• How to handle Wiberg values outside interpolation k_bondorder1-->k_bondorder2 interval?

• Add tests for fractional bond orders in the interval, and out of the interval.

• How to implement functionality in label_parameters, e.g. return the functional form or do an explicit evaluation (calculating WBOs if not present).

• How to/Should we allow mixed sections, where nonfractional and fractional are used concurrently

• Make sure simple garbage inputs do not pass tests:
  - [ ] k_bo1 and k_bo3 but not k_bo2

• Ensure that bond object order is consistent between isomorphic molecules during parameterization (needs a working isomorphic function e.g. from Josh Horton's Create OFF Mols from QCArchive #472 PR)

[codecov-io]

Codecov Report

Merging #501 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

[j-wags]

I'm taking over this effort and will open a different PR for it.