This function executes a ubuntu docker that subset a matrix
subSetCell( group = c("sudo", "docker"), scratch.folder, file, separator, cells.number )
a character string. Two options: sudo or docker, depending to which group the user belongs
a character string indicating the path of the scratch folder
a character string indicating the path of the file, with file name and extension included
separator used in count file, e.g. '\t', ','
number of cells to be extracted