Use AnnData as a component rather than a superclass of DeseqDataSet

[BorisMuzellec]

This would offer more flexibility and robustness wrt future AnnData development.

E.g., as pointed out by @grst, his would also allow storing the anndata if the user has stored that elsewhere, and using lazy loading on the h5ad which is not possible using pickle.

Beside, composition is usually better practice than inheritance.