Euler 3D examples with a test

This is work by @weslowrie (see #430, which this replaces).  It includes:

- Two 3D Euler examples: Sedov and shocktube
- A test of the Sedov problem using HDF5.

I made one important change to the Sedov problem setup: it now initializes using cell averages, rather than just checking whether the cell center is inside the initial radius.