Pandas is deprecating Panel

[moorepants]

Relevant warning from the tests:

gaitanalysis.tests.test_controlid.TestSimpleControlSolver.test_compute_estimated_controls ... /home/moorepants/miniconda3/envs/gaitanalysistoolkit-dev/lib/python2.7/site-packages/nose/util.py:471: 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.

  return func()

[moorepants]

Panel was removed in Pandas 0.25.0, see https://pandas.pydata.org/pandas-docs/stable/whatsnew/v0.25.0.html