Draft: Fix gradients of `clip` and `unitstep`, add `softclip` and `leakyclip`

Lukas Scheel-Platzrequested to merge
fix_nonlinearity_gradients into NIFTy_8

See #328 for context.

This MR adds test for full coverage of src/pointwise.py.

Edited by Lukas Scheel-Platz

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