Deprecate pyproj.transform & pyproj.itransform

[snowman2]

These are around for purposes of backwards compatibility with pyproj 1.x only. I don't plan to add anything new to them. Also, they are not optimal for use for repeated transformations. http://pyproj4.github.io/pyproj/stable/advanced_examples.html#advanced-examples

They have been superseded by pyproj.Transformer. The plan would be to add deprecation/future warnings in a 2.x release to warn users beforehand.

[snowman2]

I looked at https://pepy.tech/project/pyproj and it seems that 1/4 - 1/3 of pyproj users are still using version 1. The deprecation warnings are good enough for me for now as they let people know nothing new will be added and also give information for how to upgrade. I am going to call this issue done for now.