Recrate original object incl. data after loading model

[martinrohbeck]

Currently we are stripping the ._data attribute of the FA model before saving. I think that makes sense, to save memory.
However, when reloading the model, we should be able to re-add data to still run downstream analysis, that user proeprties from self._data. Otherwise, we have to make sure that the important downstream functionalities work w/o self._data
This currently does not work, because add_data() throws an error. @ data expert @timtreis, maybe you can have a look at this?

Feel free to use the notebook in the current PR from branch feature/issue-x/example-notebook as an example. I already added the cell, but commented it out it.

[martinrohbeck]

Save all statistics: w, z, .... in a mudata