Eigenvalue/Eigenvector computation produces warning.
During the CI build these warnings come up. I think it's ok to remove the constexpr before the if.
/duneci/modules/dune-common/dune/common/fmatrixev.hh:296:12: warning: 'if constexpr' only available with -std=c++1z or -std=gnu++1z if constexpr(Tag==EigenvaluesEigenvectors) {