New h max definition

Since `h_max` was always an annoyance, here is a better definition. 

There is a new parameter now called `max_top_level_cells` that allows you to set the (cube root of the) maximal number of top-level cells you want. So instead of referring to it as a maximal smoothing length you directly give the number of cells.
Of course if the physics requires larger cells, the parameter is ignored. This default to 12 if unspecified, giving 1728 top-level cells. 

The only reason you would want more top-level cells is for simulations on large systems to obtain a finer MPI decomposition.

I also force a rebuild after time-step 0 now to make sure we have good grid and set of tasks even if the original mesh was based on garbage smoothing lengths values entered by the user.

What do you think ?

See merge request !275