In most cases the exposure is unknown and must be learned from the data.
However, this corresponds exactly to learning the zero-mode of an
ordinary log-normal field. Hence, remove the normalization code from the
density_estimator call and drop the exposure parameter.

Adapt the density_estimator accordingly.

Re-introduce the FIXME on how the zero-mode is treated in the
`_Normalization` operator.

If no amplitude total offset (i.e. zero-mode of the amplitude operator)
is specified, the correlated field is still able to produce a useful
tensor-product of amplitudes. However, their overall zero-mode is of
course not determined. As convention, set the zero-mode of the final
amplitude operator to zero.

Adapt the statistics summary to not require the zero-mode.

Convert the return list of operators to a tuple as to disallow the
caller to amend or change it in other ways.

Since `noramlized_ampliudes` is not a property anymore, name it more
actively as to highlight the need for it to be called as a function.

The zero-mode must not be known for constructing amplitudes. Only when
finalizing the model, the zero-mode needs to be specified. Honor this by
pulling all parameters relating to the zero-mode to the `finalize`
method.

Move `moment_slice_to_average` outside of `CorrelatedFieldMaker` as it
does not depend on nor require the inherent structure of the class.

Handle the intricacies of factorizing power spectra in `finalize`
instead of relying on normalized amplitude models.

Various updates

See merge request ift/nifty!611

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

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 ift/nifty!605

correlated_fields.py: Streamline Matern parameters

See merge request ift/nifty!601

Add finufft

See merge request ift/nifty!600