Feature/restart from vtk

Fixes #458 (closed).

TODO

• Use reference vtu / vtp files that already exist for the vtkreader test
• Grid reader dim < 3
• Grid reader for vtp
• Use hasParam instead of haveParam
• If !1036 (merged) gets merged first we might need to adapt the variable names for loading the solution
• parallel (only for reading solution, grid is a different task (reading from single vtu -> ok, reading from pvtu -> new issue))
• staggered
• multidomain (we write one vtu file for each subdomain, so we can just also read it per subdomain)
• add parallel test

add multidomain test concept works but a working test could not be constructed because of the single precision issue