Shouldn't need to manually set solver.kernel_language

[ketch]

The solver kernel language could be automatically determined by inspecting the Riemann solver. Then the user wouldn't need to set it manually.

[ketch]

One way to do this is

    if hasattr(self.rp,'__file__'):
        if os.path.splitext(self.rp.__file__)[1] == '.so':
            self.kernel_language = 'Fortran'
    else:
        self.kernel_language = 'Python'

However, to avoid duplicating code this needs to live in the base solver's setup() function, and the calls to super in the child class setup functions need to appear at the beginning. I'm not sure it's worth so much trouble.