Add concept of unitsystem flags and enable overriding / extending unit systems

[adrn]

This adds a new (currently not used) set of Flag types so we can dispatch on the type of unit system (will be useful for the follow-up to add a new SimulationUnitSystem). This also adds the ability to extend or override units in an existing unit system via:

>>> usys = unitsystem(...)
>>> new_usys = unitsystem(usys, u.pc, u.erg)