Add fixed bonds and angles variable/info to xml files, and LJ/could short-range cutoffs?

[bc118]

A summary of the question or discussion topic.

Since we are adding mixing rules to the foyer XML files, do we also want to add fixed bonds and angles variable/info to XML files, designed tail corrections, and LJ/could short-range cutoffs?

For example, some of the TraPPE FF linear molecules have are semi-flexible (fixed bonds, but not fixed angles). While some of the rings are either totally fixed or totally flexible, depending on who fits the FFs.

This is not an urgent task, but it may help minimize user differences in some cases.

[bc118]

Copying @rsdefever comment over as well from this same issue in foyer.

"In GMSO I have created a FixedBondPotential and FixedAnglePotential as part of my draft PR for the Cassandra MCF writer. https://github.com/mosdef-hub/gmso/pull/560/files"

[bc118]

In our development call today, we concluded that making the fixed bonds and angles as a bool at the bond and angle level makes the most sense. This means adding an additional attribute to the force field XML files, "fixed_bond"=True/False, "fixed_angle"=True/False (or similar ... ), at the same level as the bond and angle K-constants, bond lengths, and angle degrees.