Add Toro (2009) test 2 to hydro examples.

This adds an alternative for the Sod shock test to the hydro examples that consists of a uniform initial density and pressure, with a different (non-zero) initial velocity across the central contact discontinuity. This actually creates two separate Riemann problems: a two rarefaction wave problem in the centre, and a colliding flow problem across the periodic boundary.

The solution at t=0.1 looks like this:

1D:

2D:

3D:

TODO:

• plot the solution for the colliding flows as well
• move the Riemann solver out of the plotSolution.py scripts and into a single file that can be called from all scripts
• implement a comoving version of this test
• use the correct style for plotting