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.

See merge request !3

added reading of extract-points files

varnames are read and created based on the header of the `grid.?points` files, and named e.g. `o.pt_ext_u`.

@plaa how does this look to you?

See merge request !2

added option to link auxilary files for test cases.



See merge request !1