Add wrappers for dimension reduction and gene expression pages

Use case / Idea

Provide wrapper-functions for popular R-objects that are used in the single-cell RNA-seq field: Seurat and SingleCellExperiment. Instead of requiring the user to extract and format the data that is required for the pages, write wrapper functions that perform data extraction and reformatting:

• instead of x and y coordinates of the reduced dim, the user can give just the name of the slot in the object
• instead of a data.frame with metadata, the user can give just the column names of the metadata in the object.
• instead of a count table, the user can just give the name of the assay that is stored in the object.

Implementation