Implemente pre-nonlineariaties

See merge request ift/nifty!610

Fix slice operator

See merge request ift/nifty!606

Laplace distribution operator for L1 regularisation

See merge request ift/nifty!599

Update copyright year

See merge request ift/nifty!609

Allow the SliceOperator to handle a new shape which equals the old
shape.

Ducc is optional

See merge request ift/nifty!608

Add minisanity

See merge request ift/nifty!605

correlated_fields.py: Streamline Matern parameters

See merge request ift/nifty!601

Add finufft

See merge request ift/nifty!600

add clarification to einsum docs

See merge request ift/nifty!602

Adjust the slope of the Matern kernel such that it resembles the
corresponding parameter from the non-parametric amplitude model. Change
the docstring accordingly.

Furthermore, amend the docstring of the Matern kernel to state the prior
distributions (i.e. the moment-matching) for the various parameters.