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

**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. 

