Model as dict

as per https://github.com/hyperspy/hyperspy/wiki/HSPEP1%3A-Save-models-in-the-same-hdf5-file-as-the-data

Still lacks user guide entries and tests (any ideas for them are welcome, as none similar are present at the moment)



Is there any advantage in using dill? If not, I think that it'll be better to stick to modules in the standard library for something as sensitive as saving.