fix tile size inputs

See :ref:`sec:dual_grid` for more about this approach.

When running on multicore machines with OpenMP, we can also control the distribution of 

work by setting the size of grid tiles (by defining :cpp:`fabarray_mfiter.tile_size`), and if relevant, of 

work by setting the size of grid tiles (by defining :cpp:`fabarray.mfiter_tile_size`), and if relevant, of 

particle tiles (by defining :cpp:`particle.tile_size`).  We can also specify the strategy for assigning 

tiles to OpenMP threads.  See :ref:`sec:basics:mfiter:tiling:` for more about tiling.