correlated_fields.py: Move ZM to finalize
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.