Add source/dest datum transform as member to coordinate transform and…
… change cache options accordingly
Don't remove layer coordinate transform info if layer is no longer pr…
…esent (layer could be switched off but still present)
Set higher coord_op values for ntv2 to separate them from the entries…
… coming from datum_transform.csv
Change tr() to transformation() and add python binding. Make datum_sh…
…ift db update in a transaction