BUG set normed_counts in sub_dds before recomputing genewise dispersions when refitting

[BorisMuzellec]

What does your PR implement? Be specific.

In dds._refit_without_outliers, the obsm["size_factors"] of sub_dds should be the same as the original object when computing genewise dispersions.

However, since version 0.4.1, the _fit_MoM_dispersions method (called by fit_genewise_dispersions) first checks if a normed_counts layer exists, and if not, runs fit_size_factors.

This caused size factors to be overwritten in dds._refit_without_outliers.

To fix this, this PR sets the normed_counts layer in sub_dds before computing genewise dispersions.