Add layer support to sc.tl.score_genes

[gokceneraslan]

... so that we can do crazy things like scoring velocities or scoring "unspliced expression".

[ivirshup]

@gokceneraslan I've been thinking we should have options like layer and obsm in many more places. I've started trying to implement this in a systemic way with an internal API and some test helpers in: #1173.

What would you think of using that here?

[gokceneraslan]

@ivirshup is there an elegant way to get subset of genes with _get_obs_rep? because in the current code there are things like:

    _adata = adata.raw if use_raw else adata
    _adata_subset = _adata[:, gene_pool]

[ivirshup]

Not much beyond _get_obs_rep(adata[:, genes], ...) or something like _get_obs_rep(adata.raw.to_adata()[:, genes], ...).