Units handling

[CalCraven]

Would be nice to have some more generalized unit systems that certain engines could subclass, as opposed to always calling the different LAMMPS units systems. Mostly a clarity thing.

[CalCraven]

Some clarity on this issue:
Right now we have this class:

from gmso.utils.units import LAMMPS_UnitSystems
unit_system = LAMMPS_UnitSystems("real")
outputStr = unit_system.convert_parameter(top.bonds[0].bond_type.parameters["k"])
f.write(outputStr)