multiple data layers (e.g. another matrix with imputed values)

[carsten-jahn]

Database, API and client already work with study_layer_id to distinguish multiple layers per study. For scRNA data, multiple layers get imported but only the first one is displayed in the UI.

• UI selection for layers (at least in Distribution Analysis view)
• for multiple modalities (h5mu files), the layers are imported from all modalities where they exist
• decide if multiple modalities can be in the same layer - currently it is done that way and it is convenient, but the database column study_layer.omics_type is out of line with the implementation