Feature: Add non_element_types support for atomtyping

[umesh-timalsina]

This will fix an issue @bc118 ran into while testing #644.

Currently, in #644, we don't explicitly consider the non-element atomtypes while constructing an AtomTypingRulesProvider for foyer. This PR attempts to provide that support by explicitly adding that property to the forcefield.

[CalCraven]

Few QOL changes, but I think mostly @bc118 has to test this one.

[codecov]

Codecov Report

Merging #648 (7100c02) into develop (c78e242) will decrease coverage by 0.27%.
The diff coverage is 91.66%.

@@             Coverage Diff             @@
##           develop     #648      +/-   ##
===========================================
- Coverage    90.78%   90.51%   -0.28%     
===========================================
  Files           56       56              
  Lines         4633     4585      -48     
===========================================
- Hits          4206     4150      -56     
- Misses         427      435       +8     

Impacted Files	Coverage Δ
gmso/abc/abstract_potential.py	88.88% <ø> (-1.91%)	⬇️
gmso/core/angle_type.py	100.00% <ø> (ø)
gmso/core/bond_type.py	100.00% <ø> (ø)
gmso/core/dihedral_type.py	100.00% <ø> (ø)
gmso/core/improper_type.py	95.65% <ø> (-0.19%)	⬇️
gmso/core/pairpotential_type.py	94.73% <ø> (-0.27%)	⬇️
gmso/utils/decorators.py	100.00% <ø> (ø)
gmso/utils/ff_utils.py	96.31% <ø> (ø)
gmso/core/element.py	94.82% <40.00%> (-2.55%)	⬇️
gmso/core/atom_type.py	97.18% <77.77%> (-2.82%)	⬇️
... and 8 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cd68411...7100c02. Read the comment docs.