Support units better than simple strings

[hredestig]

Summary

AssayVariables, AssayObservables have a unit which is simple string. This works sufficiently for proper units (e.g. umol / ug) but for statistics like 'log fold change' the equality functionality can be misleading.

Motivation

Statistics like 'log fold change' is relative to some other sequence so to test equality it's not enough to check they how they are named, also need to check if they are referring to the same denominator.

Proposed solution

Make unit be either a unit validated by some external library like pint or a declared Statistic with additional properties to add support for the common 'ratio to XXX'