Resample2

This patch includes the following changes, which are related to resampling.

The Image.regrid() function now uses power-of-two FFTs, because numpy FFTs, while supposedly able to handle array sizes that are not powers of 2, are extremely slow for some array sizes, particularly those that are prime numbers.

The way that window functions are applied in _antialias_filter_image() has been rewritten to guarantee circular symmetry (elliptical symmetry for rectangular pixels). More than one window function is now supported as well, although the default remains the blackman window, which is the optimal window for this use-case.

The documentation of the optional flux arguments of Image.resample(), Image.regrid() and Image.rotate() have been updated to better indicate the correct value for this argument. A flux argument has also been added to Image.align_with_image(), to be forwarded to Image.regrid() and Image.rotate().