Reduce verbosity in GMSO ForceField XML and use schema v1.1

[umesh-timalsina]

Currently, we use XML 1.0 with lxml in python. I stumbled upon a package called xmlschema which has a decent user base, that can provide a way for validation for XML schema v1.1 in python.

Additionally, the schema is quite verbose, which is understandable considering the parameters and units, but we should investigate ways to reduce verbosity of the schema.