Use a fixed, uniform cell size for hair continuum grids.

This is a bit more awkward for artists to use, but necessary for
a stable solution of the hair continuum calculation. The grid size is
defined by the user, the extent of the grid is then calculated based on
the hair geometry. A hard upper limit prevents bad memory allocation
in case too small values are entered.

Conflicts:
	source/blender/physics/intern/BPH_mass_spring.cpp

source/blender/blenkernel/intern/cloth.c

@@ -153,7 +153,7 @@ void cloth_init(ClothModifierData *clmd )
 	clmd->sim_parms->goalfrict = 0.0f;
 	clmd->sim_parms->velocity_smooth = 0.0f;
 
-	clmd->sim_parms->voxel_res = 32;
+	clmd->sim_parms->voxel_cell_size = 0.1f;
 
 	if (!clmd->sim_parms->effector_weights)
 		clmd->sim_parms->effector_weights = BKE_add_effector_weights(NULL);