Add cell_type_mapper implementation of correlation mapping

[jeremymiller]

Add a 5th mapping option in taxonomy_mapping, which is the flat version of cell_type_mapper. The steps below are a starting point for how one could do this. Inkar may have a better way.

• Precalculate a second set statistics for a flat hierarchy (e.g., only “cluster_label”)
• Store these stats in a slot called AIT_anndata$uns$hierarchical[[mode_cor]] where mode_cor = [mode]_correlation (not ideal but for back compatibility and to minimize code updates)
• Update taxonomy_mapping with a new parameter called “flat.map” that stores this version of correlation mapping and that calls cell_type_mapper with this new mode and the flat hierarchy when flat.map=TRUE
• Update documentation for taxonomy_mapping to describe the differences in all the mappings better (specifically in the header above the function, then call roxygen2::oxygenize() or however you do it).