MPI and other improvements

This merge request addresses several points:

• MPI: By using mpi.reduce instead of mpi.Reduce, it makes obsolete the need to know the array dimension on all MPI ranks.
• Array reshape of manybody.Wavefunction has been refactored to a new routine.
• Dataclasses: Remaining dependencies on dataclasses in the unittests were removed.
• Filterwarnings were adapted.