seurat day 2 solution - CreateSeuratObject warning

[kevinrue]

seurat_citeseq <- CreateSeuratObject(
  counts = umi_data_common,
  min.cells = 3,
  min.features = 200
)

produces the warning

Warning: Feature names cannot have underscores ('_'), replacing with dashes ('-')Warning: Data is of class dgTMatrix. Coercing to dgCMatrix.

For now, I've added a note to the solution notebook

> Note:
> 
> The `dgTMatrix` class represents sparse matrices in triplet format:
> - `i`, the row
> - `j`, the column
> - `x`, the value
> 
> The `dgCMatrix`class represents sparse matrices in compressed column format:
> This format stores column-oriented  data structures.
> It is more efficient for matrices with more non-zero elements,
> especially when operations involve columns.