Linear PCA hashing, Gaussian phi, spiraling chunks, log-scalogram backprop

[lostanlen]

This PR implements :

• linear hashing of multi-variable scattering coefficients and back, to treat scattering coefficients in matrix form (e.g. for PCA)
• a Gaussian low-pass filter phi, constraining S to be positive
• fixes in trim_support, untrim_support, and ifft_multiply,
• on-the-fly permtation of filtering and spiraling subscripts. this allows to make spiral scattering with chunks
• a bugfix in optimized_bank when periodized dimension has collapsed to 1
• log-scalogram nonlinearity with uniform denominator
• backpropagation of log-scalogram