Support list of parameter values in Forcefield XML

[umesh-timalsina]

Closes #674.

[codecov]

Codecov Report

Merging #681 (4b60bbd) into develop (2970412) will increase coverage by 0.01%.
The diff coverage is 87.50%.

@@             Coverage Diff             @@
##           develop     #681      +/-   ##
===========================================
+ Coverage    91.20%   91.21%   +0.01%     
===========================================
  Files           63       63              
  Lines         5160     5170      +10     
===========================================
+ Hits          4706     4716      +10     
  Misses         454      454              

Impacted Files	Coverage Δ
gmso/utils/ff_utils.py	96.01% <87.50%> (-0.30%)	⬇️
gmso/formats/mol2.py	100.00% <0.00%> (ø)
gmso/core/atom_type.py	97.36% <0.00%> (+1.31%)	⬆️

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