Streamline hdf5 improvements

The MR implements the changes in !889 (closed) without requiring the refactor of utils.

Additions:

• Option to save result/interferometer objects to pickle. This is also now a fallback in the case of generic exceptions when writing result files.

Changes:

• Make the default hdf5 saving of results use a custom serializer.
• The deepdish version is still accesible via a a hidden method, old style result files should be readable.