MNT: Deprecate Transformer.from_proj?

[snowman2]

from_proj was added based on pyproj 1 patterns of using the Proj objects with the transformer. However, with PROJ 6+ and pyproj 2+, the CRS object should be the object used when creating transformers.

from_proj does not have all of the kwargs that from_crs has and that was mainly because from_crs should be the method used and there isn't much point in supporting both. Additionally, the Proj object has some additional overhead as it creates a Transformer and a CRS when initialized.

I am thinking that removing from_proj will remove confusion.

Compatibility: from_proj was added in the initial version of the transformer in pyproj 2.1.0. from_crs was added at the same time, so if users modify their code to use from_crs instead it will also be backwards compatible.

Does anyone have good reasons why from_proj should stay?