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.

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.