Add restart test
I have added restart test to testsuite.py. When a test case has more than 1 grid level, the test case is restarted from grid level 2 with zero nsteps and then half of the iterations on the finest level are run, before another restart is carried out from level 1 to finish the simulation. The restart files are compared with a binary check. When a test case has only 1 grid level, a single restart is performed on the finest level.