Easier access to n_cpus argument in DeseqDataSet

[BorisMuzellec]

Is your feature request related to a problem? Please describe.

Following the refactoring recently introduced in #191, it is no longer possible to set the number of cpus used by DeseqDataSet when running the DefaultInference backend as DeseqDataSet keyword argument.

Currently, to set e.g. n_cpus=4 we must write something like

dds = DeseqDataSet(
           counts=counts,
           metadata=metadata,
           ...,
           inference=DefaultInference(n_cpus=4)
           )

Describe the solution you'd like

Adding n_cpus as an optional keyword argument in DeseqDataSet again, and potentially raise a warning if the inference method that is chosen cannot take n_cpus as an argument.