add functions to dicts to handle 2D dicts:

keys_of_2d_dict
fill_2d_dict
and fillgrid

and ordered() to return an ordered dict

in the ensemble.py module, ensemble_dist_to_2D_dict() converts an ensemble-style nested dataset

...->{xkeys}->{xvalue}->{ykeys}->{yvalue} = z

    to a dictionary like

    {xvalue}->{yvalue} = z

which is handy for other purposes.