Only initialise the potential after the space has been initialised

See merge request !791

Only initialise the potential after the space has been initialised as the potential requires the size of the space.

In the debugging checks for space_parts_get_cell_index_mapper() correctly ignore the inhibited particles when checking for position within the box.

In the debugging checks for space_parts_get_cell_index_mapper() correctly ignore the inhibited particles when checking for position within the box.

Basic infrastructure for black holes

See merge request !784

Do not reallocate the full sort arrays every time we sort.

Closes #570

See merge request !786