Forward-facing step example

This builds on and replaces #445.  Mainly, I've edited the comments to better document this example.  There are some unfortunate things:

- Using the Roe solver (and Classic) we get a carbuncle instability after a short time.  I understand the solution is to use an HLL solver.
- The approach we're using for internal boundaries seems to work well for Classic, but not for SharpClaw.  I think this is because the values used for reconstruction don't make sense after a while.  We could implement a different approach, in which cell values inside the step are overwritten in a `before_step` routine, but there is then ambiguity in what value to write to cells near the corner of the step.