Seperate compiletime (ie. computing) and runtime (ie. physics) variables

Regarding the SRMHD_{InitialData}.exahype files (as in 64fc9b9d), they only hold two informations:

1. Grid extends (ie. x_min=..., x_max=...)
2. Initial data to use (which does not work due to broken parser)

However, the files are highly redundant as they contain all the other information about how to compile, paths and plotters.

I really think there should be two files: One describing the application, compilation information, also the number of dimensions, polynomial order, as this are all compile time constants.

And one describing the physics, ie. the grid extend, arbitrary user constants, constants in exahype like the CFL number.

The grid definition is the only reason why users cannot proceed with their own files (we already had the discussion several times). Is there a way to decouple this?