Problem when h in ICs is >> box size

An interesting bug from a test of @zbelcheva. If the h values supplied in the ICs are much much too large, the code does not detect it and construct a TLC grid with 0 cells (!!). The check verifying we don't have less than 3 cells does not trigger. We then continue and construct zero tasks and so on.

Debugging checks trigger in space_rebuild when trying to assign the particles to the non-existent cells.

Solution: Check in regrid or before that no really crazy h values are used?

Offending test case attached (note that the check for -G when using BHs in main.c needs to be cancelled for this test) Run with --hydro --external-gravity --black-holes twobodybox.hdf5 twobodybox.yml

Overall it's a weird corner case as there is no way this 2-particle setup could ever run with SPH or anything where we do neighbour finding. Crashing more gracefully would be good though.