Add dataframe state output function

[gregmedlock]

It would be helpful to heave a helper function that returns a dataframe with rows as members, columns as features, and entries as states. Column labels should have the format:

feature.id + '_" + feature.component_attribute

This function should live in the io submodule. While making this addition, we should port the to_pickle function from Ensemble to io.

The output of this function is intended only for manual inspection and a redundant method of saving state information for publication. The output is never intended to be used as input to construct an ensemble.