Added 1D interacting blast wave test.

1D version of the Woodward & Colella (1984) interacting blast waves test. I'm still working on trying to produce a reference solution (no analytic solution exists).

I can merge that now unless you want to wait for the "reference" solution.

This can be merged now. Even if I had a reference solution, we would still need to figure out the best way to include it. I'll add it later.

Ok. I'll keep the branch alive for when the solution appears.

merged

mentioned in commit 6685fb32