Rewrite of cc.pl.enrichment

[marcovarrone]

Complete rewrite of the pl.enrichment and adding the permutation-based computation of gr.enrichment to estimate pvalues.

• Removed dependency of pl.enrichment with scanpy's dotplot.
• p-values are shown by color (parameter pvalues)
• Possibility to select a significance threshold (parameter significance )
• Show also depleted label-group pairs (parameter enriched_only)
• Show only significant pairs based on the significance threshold (parameter significant_only)