Save / load DeseqDataSet states and write anndata to disk as h5ad

[BorisMuzellec]

Is your feature request related to a problem? Please describe.
#328 makes the DeseqDataSet class unpicklable due to decorated methods. This makes it harder to save results on disk.

Describe the solution you'd like
As suggested by @grst:

implement a .save() and (static) .load() method that saves the anndata as h5ad and the state of the model as pickle or any other > suited format.

The .load() should be able to restore the full state of the dds object, including the formulaic metadata ideally

Describe alternatives you've considered

Use the to_picklable_anndata() method introduced in #328, but this loses the dds state.