A GRanges object containing a set of genomic intervals.
These will form the rows of the matrix, with each entry recording the number
of unique reads falling in the genomic region for each cell.
cells
Vector of cells to include. If NULL, include all cells found
in the fragments file
chunk
Number of chunks to use when processing the fragments file.
Fewer chunks may enable faster processing,
but will use more memory.
sep
Vector of separators to use for genomic string. First element is
used to separate chromosome and coordinates, second separator is used to
separate start and end coordinates.