Made RoI and bin-code separate functions
Clarify header comments in mkfitshdr
Added check for influence matrix calcs.
Added fit library

Used mostly for convenience functions and to remember stuff.
Forgot to import library
Bugfix for shift_img: Fourier method was broken

Fourier shifting required zero-padding before Fourier transforming,
which was omitted in this script.
Furthermore: we now use scipy.ndimage.fourier.fourier_shift to generate
the complex Fourier shift vector instead of making it ourselves.
Improved tests, also for different shifts.
Improved documentation
Make padding optional
Removed dead code