[multidomain][stokesdarcy] convenience function for decision on molar or mass density does not work for gcc 5.5

With gcc 5.5 on my laptop the introdued convenience function (commit a49c08d4) in the stokesdarcy couplingdata does not seem to work.

The error I get is:

/dumux/dumux/multidomain/boundary/stokesdarcy/couplingdata.hh:1098:52: error: use of ‘template auto Dumux::massOrMolarDensity(const VolumeVariables&, Dumux::ReferenceSystemFormulation, int)’ before deduction of ‘auto’ const Scalar rhoInside = massOrMolarDensity(volVarsI, MolecularDiffusionType::referenceSystemFormulation(), couplingPhaseIdx(domainI));

With a newer compiler version that works. @kweis: I will try to have a look at that on Friday and see if I can fix that. I think we want to support also gcc 5.5?