This reverts commit baf3a7b8, reversing
changes made to 8ce837d0.

A small ResponseOperator incl. smoothing and exposure projecting into a data space.

Increased version number to 3.0.4

- Fix serialization, total_volume and weight methods/properties in
PowerSpace
- Fix copy in LMSpace

Added domain_dtype and target_dtype attributes to FFTOperator. Removed dtype from _times and _inverse_times.

Added probing.py in demos folder.

Fixed descent_direction norming.
Extended wiener_filter_hamiltonian.py <- activated plotting.

Set default dtype for FieldArray

- Remove redundant check for axes
- Correct _distance_array_helper