This function takes a snap object and gene list as input and creates cell-by-gene matrix
createGmatFromMat(obj, input.mat, genes, do.par, num.cores)
A snap object.
Input matrix for calculating cell-by-gene matrix.
A GRanges object contains the gene annotation with name as gene name.
A logical variable indicates if run this using multiple processors [TRUE].
Number of processers to use [1].
Returns a snap objects contains cell-by-gene matrix stored in "@gmat"