Various updates

See merge request !611

Implemente pre-nonlineariaties

See merge request !610

Fix slice operator

See merge request !606

Laplace distribution operator for L1 regularisation

See merge request !599

The information theoretic interpretation of drawing samples from the
metric of the KL samples is not clear. This may be the reason why it was
not documented before.

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 !605

correlated_fields.py: Streamline Matern parameters

See merge request !601

Add finufft

See merge request !600

add clarification to einsum docs

See merge request !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.