Make scplot compatible with pegasusio

[yihming]

• In violin, heatmap, scatter, dotplot, scatter_matrix, embedding, variable_feature_plot, volcano, composition_plot, make adata be of either AnnData or PegasusIO MultimodalData type. And in code, if adata is not AnnData, convert it to AnnData.

• Add pegasusio to dependencies.

• Modify variable_feature_plot to be able to plot for Pegasus Seurat flavor highly variable genes.