Use masses defined in the forcefield XMLs instead of common elemental mass for atomistic systems

[justinGilmer]

Describe the behavior you would like added to Foyer
We should be able to use an atomistic forcefield and define masses to use in the forcefield XML file for foyer to use.

Describe the solution you'd like
This is the current behavior for non-elemental beads, but it would be useful for users so we always know which mass is being used instead of relying on some underlying inference.

[daico007]

seconded