Re-allocating neighbour list if it overflows.

[AlexanderMath]

Changing code to re-allocate neighbours instead of throwing RuntimeError('Spatial overflow.') as per discussion here.

Changes relative to discussion

1. Changed print(..) to logging.msg('Re-allocating neighbours. ')
2. Copied code in both calculator_sparse.py and calculator.py - LMK if that doesn't make any sense.

# before
if neighbors.overflow:
   RuntimeError('Spatial overflow.')
   
# new 
if neighbors.overflow:
  self.neighbors, self.spatial_partitioning = neighbor_list(positions=R,
                                                                      cell=cell,
                                                                      cutoff=self.cutoff,
                                                                      skin=0.,
                                                                      capacity_multiplier=self.capacity_multiplier)
            logging.mlff('Re-allocating neighbours. ') 
            neighbors = self.spatial_partitioning.update_fn(R, self.neighbors)
            assert not neighbors.overflow
            self.neighbors = neighbors

[thorben-frank]

thank you for your contribution @AlexanderMath :-) LGTM!