Should the unit have a unit system reference?

[pelson]

Currently you build a unit from a unit system. There is no reference to that unit system in the unit - it is entirely standalone and hold all of the information it needs to fully define the unit.

There are cases however where it is handy to have the unit system:

• Convert to could accept a string, and the unit gets created from the associated unit system (Pyudunits2 ioos/compliance-checker#1118 (comment))
• Ability to simplify 1000m into km, which needs the unit system to determine the possible prefixes etc.

We need to decide whether the unit can hold on to the unit system, or if we put such functionality down into the system.