Evrard and other tests

Added Evrard collapse test and made sure it works with GIZMO. Added some other tests: 3D Gresho vortex, Kelvin-Helmholtz growth rate tests (which do not work with single precision accuracy), spherical 2D and 3D Sod shock, vacuum expansion test in 1D, 2D and 3D, and 2D and 3D spherical vacuum expansion test. Added scripts to retrieve reference solutions for interacting blast waves test, Evrard collapse and all spherical tests.