Input sanitization

[kieranrcampbell]

In the main function you'll need to

1. Check that the input is a SingleCellExperiment - could also reasonably be a count matrix?

2. Be nicer about user clusters. Currently has
   clusters <- lapply(clusters, as.vector, "numeric")
   but what if this is instead a character vector of cluster names? What happens if some cells aren't assigned clusters (ie there are NA values)

3. The clusters currently have to be "A list of clusters, an array or list of integers of same length as number of cells in sce." - what's the difference between a "list of clusters" and a "list of integers"? Also it starts with lapply (see above) - with this work with an array?

[kieranrcampbell]

Similarly, you'll want to check the dimensionality of the clusters matches the input SCE