Add utility for constructing the operators used by a discretization emplying RV-stabilization - Add constructDiffOps which creates the operatores used for a scheme with RV-stabilization - Minor clean up in RungekuttaExteriorRV

# SBPLIB
sbplib is a library of primitives and help functions for working with summation-by-parts finite differences in Matlab. To use sbplib download the code and add the sbplib folder to the matlab path.