Allow reprojecting the vertical dimension via vertical CRS

[asinghvi17]

This is not immediately necessary but would be cool to have - it would simply be special treatment for the z-dimension, as we already have for x and y. Could also do something for time at some point.

What do you think? Some CRS do define a vertical CRS as well, usually to serve as a reference point. Would be nice for reproject to handle that.