Clean coreg.py

[adehecq]

There seem to be several unused functions in coreg.py, e.g.:

• get_terrainattr
• rio_to_rda
• mask_as_array -> used only once in ddem.py...

It would be good to remove those if they are indeed not used anymore. @rhugonnet and @erikmannerfelt, if you give me the go, I can delete/move them as part of my next PR.

[rhugonnet]

Agreed! The first two functions, I moved and updated into terrain.py in #227, they are now used there.

[adehecq]

Sorry, I did not realize that I did not look at the latest commit and the richdem related functions had already been moved !