Refactor into CoordinateTransformationInterface.jl and CoordinateTransformations.jl

There are some cases in which it would be nice to have the abstractions of coordinate transformations without the overhead of StaticArrays and LinearAlgebra.

I could refactor to create a lightweight CoordinateTransformationInterface.jl as a subpackage here that folks can depend on, at least for API access.  Would that make sense to folks here?