store SI-prefixability metadata in the unit registry

[ngoldbaum]

Rather than keeping a global list of units that can be SI-prefixed, let's store that data in the unit registry. That means adding that data to the units in the default unit symbol lookup table. It also means adding the ability to add new SI-prefixed units to a unit registry. It was already possible to do that globally via define_unit, now it's possible to do it just for a single unit registry.

This makes it easier to use unyt as a library without messing with unyt's internals. It also eliminates one vector for cross-library interference by no longer having as much global state.

I found this while porting yt.units to use unyt under the hood.