output total electron density

- NEW : output total electron density
- FIX : fig a bug in updateRhsForTrapping when using explicit method
some revision when testing the charge balance

- NEW : add the method of verifying the solution of Ax=b
- NEW : add the method of writing matrix to file
Revert all the aggressive changes for test charge balance

- ENH : remove some commented lines
- CHG : change the ordering method in the sparse solver
- NEW : add the iterative solver for solving sparse matrix (commented)