Rotate projection images

Asked by @ludwig.

Let the reference frame be as follows, with the origin on the sample stage:

The rotation axis can be tilted by a solid angle spanned by two planar angles (alpha, beta):

1. Planar rotation around the axis x with angle alpha
2. Planar rotation around the axis y with angle beta

Case (1):

Case (2):

The generic solution is to account for this tilt in the backprojector.
In case (2), however, things can be handled by first rotating the projection images. The filling mode can be set to "edges" (see scipy documentation) to handle local tomography.