Provide other padding options in paganin_filter_tomopy

[yousefmoazzam]

Just to improve the visibility of this TODO I saw in a code comment so it's less likely to get missed:

httomolibgpu/httomolibgpu/prep/phase.py

Lines 303 to 305 in ce1c15f

	# Perform padding to the power of 2 as FFT is O(n*log(n)) complexity
	# TODO: adding other options of padding?
	padded_tomo, pad_tup = _pad_projections_to_second_power(tomo)

[dkazanc]

I guess there are 2 things, padding in a sense of extending the area and also filling the padding area with some values. This comment refers to the former but it could be the latter that it is more important. Apparently, padding with zero values can be not the best choice.
Coming back to this issue, I'm not sure what exactly we gain if we do the padding area smaller for instance. It requires a bit of benchmarking first, but I think initially this was a comment to deal with the fact that in TomoPy they do not scale to the power of 2. Which is good when the memory is in question.