Changed padding strategy, pad by PSF size first then to next highest product of 2,3,57

[VolkerH]

With this branch I changed the padding strategy to address GPU memory wastage and wrap-around artefacts.

For background see the dicussions here:
#31

New padding approach:

• width of the support of the PSF is calculated for each wavelength and the maximum along each spatial dimension is taken as the minimum padding value for that dimension (passed to flowdec as pad_min)
• the resulting shape is padded to the next product of factors 2,3,5,7 along each dimension. This is done via changes introduced to flowdec in this branch https://github.com/VolkerH/flowdec/tree/padcufft which are not present in the upstream flowdec repo (pull request pending)