Use latest release of GMSO; add GAFF to tests

[chrisjonesBSU]

We should change our environment files to use GMSO from conda rather than source, and test that the changes made in GMSO for GAFF are working.

[chrisjonesBSU]

Note: For the unit tests to pass, the systems will have to be molecules that don't contain improper dihedrals as the GAFF impropers are not yet supported by hoomd.