Speedup to_parmed conversion

[daico007]

Address #766. A few minor changes that help speed up the to_parmed conversion.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (24a6101) 91.89% compared to head (d1d78ab) 91.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #768      +/-   ##
==========================================
- Coverage   91.89%   91.88%   -0.01%     
==========================================
  Files          66       66              
  Lines        6733     6732       -1     
==========================================
- Hits         6187     6186       -1     
  Misses        546      546              

Files Changed	Coverage Δ
gmso/external/convert_parmed.py	95.09% <100.00%> (-0.02%)	⬇️

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[CalCraven]

LGMT if this offers enough speedup! Do we want to do the same for connectiontypes, or is it not impacting things as much?

[daico007]

all the connection types are actually using a different filter (pfilter = PotentialFilters.UNIQUE_PARAMETERS), so that might have already been included.