Image planes can mismatch the desired sampled region

[fjebaker]

If the outer radius on the image plane is not a few factors of maxrₑ, which may occur if a user is specifying the plane, then there is no warning that the sampled region may not actually be the desired region.

The image_plane function should be reworked, so that the planes themselves store parameters of the plane, but the size of it is scaled internally, so that this mistmatch cannot occur.

This should also still be useable in the rendergeodesic functions however, so some care in the design needs to be taken.

[fjebaker]

Same applies for the inner radius, especially in the context of obscured discs.