[FEATURE REQUEST] visualization of multimodal data in mudata (h5mu) format

[bsiranosian]

Is your feature request related to a problem? Please describe.
I love cellxgene for visualization of scRNAseq data. However, only a single modality of data is supported. Given the increasingly-multimodal nature of single cell experiments, I want to be able to visualize data where the same cell was profiled with different modalities. For example, with 10X Flex, you can profile RNA and protein abundance in the same cells.

Describe the solution you'd like
Cellxgene would support multimodal data in the mudata (h5mu) format. Mudata provides a multimodal wrapper of anndata objects.
Upon loading an h5mu file, cellxgene would display information in the first data modality. Users could pick which modality the PCA/UMAP display would be drawn from. Users could also color the plot with values drawn from different modalities.

Describe alternatives you've considered
A low-effort way to implement this would be to prompt the user which modality they want to visualize in the browser, and limit to only data present in that modality. The data structure for a single modality is just an anndata object, so cellxgene would function as normal.