Fixes for fftflip and gaussian image filter

Jens Luchtrequested to merge
fixes into master

hotopy.fourier.fftflip keeps requires_grad attribute if called with PyTorch tensor.

Fix hotopy.image.filter.gaussian_bandpass_real behavior for last axis with odd length.

BacktrackingPGM now has easy accessible residuum and relative_residuum properties.

Edited by Jens Lucht

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