Expanded search for improper_types

[umesh-timalsina]

For an improper, if we are to consider atom1 as the central atom, then the order of atom2/atom3/atom3 really doesn't matter when it comes to finding a matching type from the forcefield. In #644, when testing on a system with impropers, I found that we were only trying to find a potential match using two possibilities, while there should have been 6. This PR is an attempt to fix that.

Previously, the following code would error out:

>>> ff_with_impropers = ForceField()
>>> ff_with_impropers.name = "imp_ff"
>>> ff_with_impropers.improper_types = {
    "CT~CT~HC~HC": ImproperType(name="imp1"),
    "CT~HC~HC~HC": ImproperType(name="imp2"),
 }
>>> ff_with_impropers.get_potential("improper_type", ["CT", "HC", "HC", "CT"])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/umesh/krow/mosdef/gmso/gmso/core/forcefield.py", line 270, in get_potential
    return potential_extractors[group](key, warn=warn)
  File "/home/umesh/krow/mosdef/gmso/gmso/core/forcefield.py", line 455, in _get_improper_type
    raise MissingPotentialError(msg)

With this branch, we are able to find the correct parameters for improper based on the central atom regardless of how the rest of the atoms are ordered.

[codecov]

Codecov Report

Merging #665 (ae5b1de) into develop (02bdcd3) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #665      +/-   ##
===========================================
+ Coverage    91.08%   91.10%   +0.02%     
===========================================
  Files           57       57              
  Lines         4756     4758       +2     
===========================================
+ Hits          4332     4335       +3     
+ Misses         424      423       -1     

Impacted Files	Coverage Δ
gmso/core/forcefield.py	93.47% <100.00%> (+0.41%)	⬆️

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