Fix KeyError in rank_genes_groups_violin with gene_symbols argument
Fix KeyError in rank_genes_groups_violin with gene_symbols argument

When using the gene_symbol argument, KeyError is raised accessing adata.var when it should be accessing adata.raw.var. The variable, g, is also reassigned so df column names match new gene symbols when calling pd.melt on df.
theislab/scanpy,theislab/scanpy