Unit / Dimension concurrency warnings. Add Sendable conformance?

[tcldr]

I'm not sure of the best place to surface this, but as mentioned on this Swift forums post it seems to me that the explicit, unavailable Sendable conformance of Unit and Dimension, is incompatible with the stated usage of Unit and Dimension.

Specifically, in strict concurrency 'complete' mode, the Swift 5.10 compiler flags the creation of static properties with the warning 'Static property 'x' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor'. Of course, Isolating properties to an Actor isn't feasible either as not only does being tied to an Actor limit the utility of the class, but the the properties become inaccessible to the essential baseUnit override.

Perhaps this is something that #237 will address?