Adopting the GRMHD specfile for the 2D experiment.

For some reason, the C++ parser doesn't like this spec file any more:
What happens is an infinite timestep and a parser error afterwards (why
is the spec fiel parsed after the first timestep anyway?)

 0.0459348    info         step 0       t_min          =0
 0.0459414    info              dt_min         =1.79769e+308
 0.0459557    info              memoryUsage    =65 MB
 0.0459649    error        Invalid simulation end-time: notoken
(file:/home/koeppel/numrel/exahype/Engine-ExaHyPE/./ExaHyPE/exahype/Parser.cpp,line:377)

Strange bug.