mesh overallocates memory for 1D and 2D simulations

[natj]

The internal grid inside tiles (Mesh) allocates halo regions in the y and z directions for 1D/2D simulations.

Fixing this would be easy in the Mesh allocator by checking for Ny==1 or Nz==1,, but some Python code relies on being able to loop over indices in the full range from -H to N+H, where N is the mesh length in the y or z direction and H=3 is the default halo size.