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)



Ok, it makes sense - if a "checkpoint" dictionary is needed, it can be deepcopied afterwards. Also, slicing at the moment results in shallow copies - however it is not relevant here (again, wrong PR) :)