possible division by zero

[rheiland]

It is not critical as it will only happen in unusual model domains, but in pyMCDS.py, in get_mesh_spacing, it is possible to get a runtime error due to division by zero on:

        dm = (tr_m_range[1] - tr_m_range[0]) / (ar_m_axis.shape[0] - 1)
        dn = (tr_n_range[1] - tr_n_range[0]) / (ar_n_axis.shape[0] - 1)

I encountered it while writing what was essentially a unit test. But I found a workaround, so it's not critical for me. Just FYI.