I had to remove some documentation since it was only copied from another
class and did not fit to the class at hand.

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.

Introduce a SliceOperator for slicing a field into a smaller size all
while preserving distances of the domain. Optionally center the selected
slices.

Plus, introduce a SplitOperator for splitting a single Field into a
MultiField.

Test said operator for a couple of cases inspired by real-world
scenarios.