dont overwrite chk files if the solver was restarted

in case f a restart, read the number from the restart file and use this as starting point for the next chk files, so that when restarting from foo_215.chk, the first chk file written is not foo_0.chk but foo_215_0.chk (which contains the initial conditions and should in most cases be identical to foo_215.chk). The next file will be foo_216.chk with the new timestep.

In case foo_215_0.chk already exists, we write foo_215_1.chk and so on.

The filters still overwrite everything, maybe we should change that too?

See merge request !707