EulerEq: Fix 2d equations and timestep scaling

• The 2d version did compute the flux incorrectly.
• We used too strong of a scaling for the time step, e.g. N^-3 instead of N^-2. This is fixed for 1d and 2d case.
• Updated euler2d_isentropic_flow ref test to no longer run into a shock, because that example doesn't use any HRSC.
• Updated euler_sod_shock_tube test to run with low resolution. Also shorten tend so that data does not reach boundary.