Shift finding for translation stage along beam

Implements the fitting of the detector transverse movements, when displaced along the beam direction.

Main tasks

Reference octave function can be found here: https://gitlab.esrf.fr/tomotools/octave_archive/-/blob/master/m/tomotools/alignxc.m

To be done:

• Refactor alignment module -> re-use of code
• Implement optional band pass filters
• Implement bi-quadratic regression of stationary point
• Translate linear regression of translation shifts
• Create Unit tests
• Add documentation, examples and FAQ to dosctrings

Update June 15th 2020.

The following items have been added to the discussion / TODO list:

• Convert the docstring style of the low pass function.
• Decide where to put the low pass function (here as a hidden function, or a dedicated module in nabu.misc?).

Optionally:

• Add tests for the peak fitting functions.

Deferred to a future MR:

• Add checks for the shift overflow in the correlation output: pixels in the center of the image, crossing the boundary from left to right / top to bottom.

Update June 23rd 2020.

The following items have been added to the discussion / TODO list:

• Add description on how to convert the returned values into the unit of usage (e.g. renormalize by pixel-size), to docstring of DetectorTranslationAlongBeam.find_shift (this solves one of the items above).
• Set consistent typing throughout all alignment functions (as per: !41 (comment 76964)).
• Use scipy.fftpack and rfft (as per: !41 (comment 76964)).