Specify var subsets for stats testing

Hi authors,

First off, love scanpy. Big fan.

I was just wondering if you have considered including an option in scanpy.tl.rank_genes_groups to specify which variables to select for testing, allowing users to select a subset of variables which would or would not be considered in the statistical test.

For context, I'm trying to test between groups of cells while ignoring ribosomal / mitochondrial genes, but retain them in the .var and .X objects for downstream analysis/visualisation. Making a temp object with these variables removed solely for stats testing partially works, but it's confounded by having to further apply the boolean slice to the .raw object as well.

Thanks, K

Start with the scanpy.tl.rank_genes_groups entry point and its documentation, tracing how variables and the .raw object are selected for testing. Clarify the intended subset semantics and identify the relevant tests; done means users can exclude or include selected variables for statistical testing without removing them from .var, .X, or downstream visualization data.