Merge branch 'super_cell_fix' into 'master'

Set the super-cell to the first cell with interactions.

The super cell is either a non-empty leaf or the first cell for which an interaction task is defined. need to add a condition for gravit tasks once these have been settled.

Separating this from the `mpi_skip` branch as it is not really related and should be tested separately.

See merge request !186