Internalize "known units"

[prusse-martin]

Add a internal list of "known units" instead of accepting all available in barril (or or auto cover those "unknown units" to the base unit when used in order to be compatible with the released version of alfasim).

Rationale:

Following the loosening of the dependency o barril (#326) users of the sdk could end up using a more recent version of barril (one that add new units) making it possible to create alfacases (and possibly plugins) that rely on units unknown to alfasim making the incompatible.

As of today the only place those "known units" are listed is in the documentation (like the "Available units for category ‘time’" section in the online documentation).