New Violin Plot Function (analogous to scanpy: `sc.pl.stacked_violin` with identity on y axis

[samuel-marsh]

Create a new violin plot function that allows for plotting of vertical violins with identities stacked on the y axis and features on the x axis. Colored by continuous gradient based on gene expression in identity.

Example: Figure 2E: https://www.science.org/doi/10.1126/science.abl5197#F2

Code: https://zenodo.org/record/6334988 TICA_04_myeloid.ipynb sc.pl.stacked_violin(migDC_subset, var_names=['CRLF2','CCL17','CSF2RA','GPR157','CCL22','SPRED2','EBI3','AIRE','PDLIM4','ROR2','LZTFL1'], figsize=(8,3), groupby='organ_code',title='Migratory DCs', cmap='GnBu', standard_scale='var', categories_order=['LNG','LLN','MLN'])

• [x] New fetch and organize data function
• [ ] Test best value to use for fill
• [x] Enable aesthetic stacking to match Stacked_VlnPlot
• [ ] If possible wrap into current scCustomize VlnPlots with logical parameter (identity_y_axis?). (Need to ensure it is not confusing with flip parameter that creates hortizontal violins.