Updates to NAGL partial charge handling

[timbernat]

Updates to the OpenFF toolkit NAGLToolkitWrapper have made using the NAGL GNN charge model easier than ever, and have provided newer versions of this charge model which are publically available. The polymerist wrappers on top of this are in need of a handful of updates in turn, namely:

• Deprecating the manual load of a GNNModel object in favor of openff.toolkit.utils.nagl_wrapper.NAGLToolkitWrapper
• Deprecating the auxiliary openff.nagl_models import
  • Making the nagl dependency entirely optional, and adding an asserting for the presence of the openff.nagl package to those modules which require it
• Updating the required version of openff-nagl to 0.4.0 in conda dependencies
  • Updating the default model used by the NAGL MolCharger subclass from RC2 to RC3