Version string crashing

At least with ADERDG Limiting applications, when calling --version, we get a crash:

$ ./ExaHyPE-MHD  --version
 0.000146866  12:08:29     [sveinn]rank:0 info         tarch::parallel::Node<static>::reserveFreeTag()         assigned message JoinDataBufferPool[vertex] the free tag 0
 0.000248671  12:08:29     [sveinn]rank:0 info         tarch::parallel::Node<static>::reserveFreeTag()         assigned message JoinDataBufferPool[cell] the free tag 1
 0.000282049  12:08:29     [sveinn]rank:0 info         tarch::parallel::Node<static>::reserveFreeTag()         assigned message JoinDataBufferPool[cell-marker] the free tag 2
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid
Abgebrochen

This is certainly because the Solver toString() methods print a lot of information and somewhere there is a nullpointer inbetween, probably the linked FV solver in the limiting scheme.

I don't like to have such a verbose toString() anyway, but we'll see. Maybe I put in a more decent toString() method.

The toStrings also lack the name of the actual class, ie. the new output how Dominic created it is no more able to tell about the actual class names.