Identifier key as part of potential type

[CalCraven]

Adding the string class "identifier" to potential objects, such as angletypes, bondtypes, etc, so these can be stored such that rewriting a forcefield using "classes" or "types" is possible.

Use Case

import gmso
ff = gmso.ForceField()
bondtype = gmso.core.bond_type.BondType(name="b1", identifier="CT-CT", member_classes=("CT", "CT"))
ff.bond_types["CT-CT"] = bondtype # this key and the identifier string should match
# TODO: probably would be nice to set it up in gmso so you can only specify `member_classes`, and `identifier` will be populated from there.
ff.to_xml("tmp.xml")
load_ff = ForceField("tmp.xml", strict=False)
print(load_ff.bond_types.keys())
print(hasattr(list(load_ff.values())[0], "identifier"))

With PR, output is

"CT-CT"
True

Without PR, output is

"CT~CT" # Note this is changed from - to ~ as the bondtype, since the ffkey is generated from member_classes, which loses info on the bondtype.
False

**Note this requires a different version of gmso to run. Will need mosdef-hub/gmso#958

[CalCraven]

Wait to merge until mosdef-hub/gmso#958 is ready to go.

[CalCraven]

Probably need to hard and fast map the identifier to a tuple of values instead of the string. I.e. it should be ("CT", "~", "CT") instead of "CT~CT". This would help us handle the case with bond symbols in the atomclass names, since then they could be specified as type1 = "C=", and type2="C=", which maps to ("C=", "=", "C=")

[codecov]

Codecov Report

❌ Patch coverage is 8.47458% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.69%. Comparing base (529839d) to head (c067615).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
forcefield_utilities/gmso_xml.py	8.47%	54 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (529839d) and HEAD (c067615). Click for more details.

HEAD has 160 uploads less than BASE

Flag	BASE (529839d)	HEAD (c067615)
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Additional details and impacted files

@@             Coverage Diff             @@
##             main      #64       +/-   ##
===========================================
- Coverage   93.29%   76.69%   -16.61%     
===========================================
  Files          12       12               
  Lines        1641     1665       +24     
===========================================
- Hits         1531     1277      -254     
- Misses        110      388      +278     

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