More consistently handle unit string constants

[tylerflex]

For example, what if we have an inverse micron, or a more complicated unit? We could standardize how we display the units and also perhaps replace the constants with just a set of rules to follow when defining units directly in the fields? eg.

Use abbreviated form of SI units, eg. (ps, nm).
Do not use greek letters (eg. um not μm)
Use asterisk to define multiplication (eg `um * N` not `um N`)
put parentheses when dealing with complicated fractions, eg (um * N)/(ps * kg)
For inverse units, use 1/x or x^(-1)