2D / 3D build check is not performed correctly

We have a startup check in the code that makes sure that the specfile has some same const build constants as for instance the ADERDG polynomial order. However, this test fails for checking that for instance a 2D build is also runned with a 2D grid specification. Thus we get strange errors as in 10b1261b :

 0.0459649    error        Invalid simulation end-time: notoken
(file:/home/koeppel/numrel/exahype/Engine-ExaHyPE/./ExaHyPE/exahype/Parser.cpp,line:377)
 ` ``

And that not after the beginning but after one timestep.

There should be a well-speaking check instead at the beginning.