Support sparse matrix storage

[scjlee]

If we want to further extend the usability for single-cell data. It might be good to have an option to store data field as a sparse matrix.

This might need to rewrite some functions/methods to use implementations from Matrix package and/or RcppArmadillo. This will help NOT converting data to dense matrix class in R.

Some might-be-useful links:
https://cran.r-project.org/web/packages/RcppArmadillo/vignettes/RcppArmadillo-sparseMatrix.pdf