MAINT: more refactoring of downsampling_convolution
MAINT: more refactoring of downsampling_convolution

Make `j` monotonically increasing for all loops, makes it a little easier to
see, especiall w.r.t. next commit.

michelp/pywt,rgommers/pywt,grlee77/pywt,ThomasA/pywt,rgommers/pywt,PyWavelets/pywt,ThomasA/pywt,aaren/pywt,eriol/pywt,michelp/pywt,ThomasA/pywt,PyWavelets/pywt,grlee77/pywt,kwohlfahrt/pywt,aaren/pywt,kwohlfahrt/pywt,kwohlfahrt/pywt,rgommers/pywt,eriol/pywt,eriol/pywt,aaren/pywt,rgommers/pywt,michelp/pywt