fix padding in rescaling to use reflect correctly

[McHaillet]

The imaged I attached in #63 showed a strong edge artifact, which I first though came from shifting. Turns out it originates from rescaling. Apparently the padding with zeros during rescaling created lines in my tilt-series, as visible in the top left corner of this image:

I made a small fix in _pad_to_best_fft_shape_2d as it was not correctly switching to 'reflect' mode. Also a suggested change to pad images with the image mean as the constant value for too much padding to still somewhat reduce edges. Now rescaling looks like this:

And it edges after shifting are indeed reduced:

[alisterburt]

Amazing! Thanks for catching these

One minor nit, let me know what you think we should do about that but otherwise let's get this in!

[alisterburt]

Having trouble merging this from mobile, not sure what's up - will sort when back at a PC unless you want to rebase yourself?

Edit: ignore above