REFACTOR: make optimized spline calibration backward compatible

I noticed that the recent changes to the spline calibration model mean that calibration objects pickle dumped in Bilby<2.1 are incompatible with Bilby==2.1.0. This is causing some failures for me with bilby_pipe. This MR does some refactoring of the new attributes to make sure it is backward compatible.

As another thought, this happens reasonably frequently, we should look into ways of testing that various file formats written with the old version are compatible with the new version before making releases.