add projection to get_coords

[FabianHofmann]

Should we add the concept of a projection into this get_coords function? Think rotpole transform for cordex for example.

I'd propose to have this as a separate issue, though!

Originally posted by @coroa in https://github.com/PyPSA/atlite/diffs

[euronion]

I think it is a reasonable extension. But no need to include it in v0.2. Let's leave it an enhancement for a later release.