Fix rounding error bug in calculation of number of top-level cells

Because a value of 0.99 was used to force rounding down (see changelog), trying to use a top-level cells count of ~100 could lead to rounding to +/-1 from expected value. This more adaptive approach should avoid this issue for arbitrarily large numbers of top-level cells (at least until Ncells^2 approaches the floating point limit).

There is a potential issue if trying to use one top-level cell. I think it might make sense to enforce a minimum value for the tol that I've introduced (could be the previously-used 0.99, for instance, just needs to be less than 1). Thought I'd ask here before adding more logic, though.

src/space.c

@@ -1231,7 +1231,11 @@ void space_init(struct space *s, struct swift_params *params,
   int maxtcells =
       parser_get_opt_param_int(params, "Scheduler:max_top_level_cells",
                                space_max_top_level_cells_default);
-  s->cell_min = 0.99 * dmax / maxtcells;
+  /* We're at risk of rounding errors if tol ~ 1 - 1/maxtcells. */
+  /* Make sure it's (much) closer to 1.0 than this. */
+  /* But also ensure it's not so small that we round in the other direction */
+  const float tol = max(1.0 - 1.0 / (maxtcells * maxtcells), 0.99);
+  s->cell_min = tol * dmax / maxtcells;
 
   /* Check that it is big enough. */
   const double dmin = min3(s->dim[0], s->dim[1], s->dim[2]);