More robust and concise OpenFF MolCharger dependency management

[timbernat]

The current dependency structure and attribute registration of the mdtools.openfftools subpackage is somewhat of a hodge-podge, and to an extent reinvents. Also, for the future it would be nice if all features of polymerist which require parts of the OpenFF stack could be sequestered and controlled within this subpackage, to better control dependencies and allow for the OpenFF dependency to be optional; this is a major barrier to registration of polymerist on PyPi.

• Implement checkers and decorators for enforcing package requirements (similar to openff.utilities)
  • Add support for custom Exception raised for missing packages
  • Add support for wrapping around methods as well as functions
  • Write units tests for these
• Separate ToolkitWrapper registration case-by-case with import-awareness
  • Make unit tests aware of missing packages (should not fail due to missing package or license, but reduce number of tests)
  • Recast MolCharger in terms of register_abstract_class_attr decorator
  • Have MolCharger implementations rely directly on respective ToolkitWrappers, rather than on the GLOBAL_TOOLKIT_REGISTRY
• Write unit tests for MolCharger subclasses
  • Method name application
  • Partial_charge attr creation