Issue with corner plot with multiple files

@charlie.hoy, I'm seeing weird behaviour when I make comparison pages between Bilby and LALInference results, e.g. here. I suspect this is due to an indexing problem.

Two possible fixes would be:

• When loading multiple results make sure the common parameters are stored in the same order.
• Use pandas DataFrames to store the posteriors internally. This would enable easier indexing by parameter names.

I would vote for the second, I'm happy to take a look at doing this, if this is the desired change.