add forward step problem

This pull request implements 2D flow over a forward facing step, using the compressible Euler equations.  The new thing it involves is the presence of boundaries that are internal to the domain.  The implementation is done by using an aux field that is set to 0 in the step and to 1 elsewhere.  Then in the Riemann solver, when the left and right states have different aux values, the solution values in the step region are overwritten with those for a reflecting boundary.
I might suggest making the module something more descriptive than `riemannsolver` as there might be need for more of the eventually.