add `Dune::IsNumber` trait for ADOLC's `adouble`
requested to merge burchardt_at_igpm.rwth-aachen.de/dune-fufem:bugfix/IsNumber-for-adouble into master
This is required for, for example, FieldMatrix<adouble, n, n>::invert()
.
This is required for, for example, FieldMatrix<adouble, n, n>::invert()
.