Remove use of pandas Panels (now deprecated)

[mhearne-usgs]

Deal with the warning below. I think the best way to do this is to change the use of a Panel to a dictionary of dataframes.

/Users/mhearne/miniconda3/envs/pager/lib/python3.5/site-packages/pandas/io/pytables.py:734: FutureWarning:
Panel is deprecated and will be removed in a future version.
The recommended way to represent these types of 3-dimensional data are with a MultiIndex on a DataFrame, via the Panel.to_frame() method
Alternatively, you can use the xarray package http://xarray.pydata.org/en/stable/.
Pandas provides a .to_xarray() method to help automate this conversion.