Add option to use area-of-interest in to_vcrs()

[rhugonnet]

Optionally, the pyproj.transformer.TransformerGroup used in vertical transformation can refine the grid it is using by being passed an AOI (which could simply be the DEM.bounds or EPC.bounds).

It should remain an option that is not the default, as it is less commonly used and could result in wrong back-transformation for classic ellipsoid to geoid grid conversions used by most users.