Add ability to specify units

[edmundsj]

Currently, the default units for internal databases are microns. If these materials are not used, the units of length can be chosen to be anything (meters, nanometers, flargons), as Maxwell's Equations are length-invariant. However, some users may prefer to explicitly specify units for their simulation. This would allow materials dispersion to be specified in terms of the user's preferred units, and would make plots easier to interpret

Proposed Implementation

solver = Solver(stack, source, units='nm')

This would require some slight refactoring of materials lookup, but would allow future-proofing to work with materials databases outside the optical range.