Add embedding-coordinate geometry functions & output

[bprather]

This PR adds functions for the metric in whatever the embedding coordinate system is. gcov_embed was already present, but we add gcon_embed and gdet_embed for completeness. Note we don't cache these, just add the functions for user/package code.

Also added the option to output the _embed versions when creating coordinate/geometry outputs, which might be useful for new/complex metrics which may be a pain to re-create in code reading the files.