Add useful geometry utilities

- [ ] `change_resolution()` #% For a given geometry, returns a lower resolution version of it. For FDK only image needs to be changed, but for interatives also the projections, maybe this function can also resample your projections?
- [ ] `compute_voxel_size()` #% Voxel sizes of a real dataset should generally be geo.dVoxel=geo.dDetector*geo.DSO/geo.DSD; Maybe we can have a function to do so, such that users don't need to guess. This function can perhaps also return a reasonable image size and number of voxels. maybe `compute_image_geo()`