name index columns in DataFrames returned by FitGrids

[andportnoy]

grid.params

returns a DataFrame with an index column of parameter names, but the index column itself is not named. It would be helpful to have these index columns named:

1. If we get a single index column, name it using the parameter name used to query the FitGrid (e.g. params)
2. If we get multiple index columns, name them using the parameter name + index subscript.
3. If any of the index columns are named already, don't touch them.

[andportnoy]

We don't support multiindices anyway, so that makes life easier (cross out number 2 above): https://github.com/kutaslab/fitgrid/blob/282ff50b39d28d02fce633c7f376e64bdc9b1eb2/fitgrid/fitgrid.py#L211-L215

[andportnoy]

Nope, don't cross out number 2. When we get a DataFrame, we stack the columns into long form, which creates a MultiIndex.

[andportnoy]

closing for now