Updates to steerable pyramid

[billbrod]

This PR has two small updates to the steerable pyramid:

• We had previously raised an error when order=0, which was due to a reconstruction error. Looking into it, this error only happens for the complex pyramid and we now just raise a warning (as for odd-sized images, which have the same problem).
• We do, however, raise an exception if the user tries to create a complex pyramid with order=0 that is tightframe, since that is impossible.
• Furthermore, we add a small section to the steerable pyramid notebook showing how to use torch / torchvision to pad or pad and crop, in order to change the behavior at the boundary.

There's a question about where the warning about reconstruction belongs: during pyramid object construction or during the call to the method that actually does the reconstruction (recon_pyr)?

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...e/canonical_computations/steerable_pyramid_freq.py	87.50%	1 Missing ⚠️

Files with missing lines	Coverage Δ
...e/canonical_computations/steerable_pyramid_freq.py	91.27% <87.50%> (-0.18%)	⬇️

[hmd101]

Looks good to me!

[BalzaniEdoardo]

Looks good to me, and I agree that the error should be raised at initialization.