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)



a) I agree, in model fancy indexing it is a reasonable thing to only make a shallow copy (it does not at the moment, I'll change it)
b) Here we are not slicing, just constructing a dictionary of the model (a different PR).