Implementations for geometryInInside() and geometryInOutside()

This implements said methods geometryInInside() and geometryInOutside() for SubgridLeafIntersections. While the concept of creating new geometries via Dune::MultilinearGeometry where needed was rather straight-forward, the actual implementation took quite a bit of work due to the different types. @akbib and @maxka came up with using C++17's std::variant and @akbib did the work implementing this. I contributed a fallback implementation of std::variant that does not need C++17.

dune/subgrid/subgrid/subgridgeometry.hh

@@ -233,12 +233,12 @@ class SubGridLocalGeometry
 
         //! The Jacobian matrix of the mapping from the reference element to this element
         const JacobianTransposed jacobianTransposed (const FieldVector<ctype, mydim>& local) const {
-            return Std::visit([&](auto&& geom) {return geom.jacobianTransposed(local);}, localGeometry_);
+            return Std::visit([&](auto&& geom) {return (JacobianTransposed) geom.jacobianTransposed(local);}, localGeometry_);
         }
 
         //! The inverse of the Jacobian matrix of the mapping from the reference element to this element
         const JacobianInverseTransposed jacobianInverseTransposed (const FieldVector<ctype, mydim>& local) const {
-            return Std::visit([&](auto&& geom) {return geom.jacobianInverseTransposed(local);}, localGeometry_);
+            return Std::visit([&](auto&& geom) {return (JacobianInverseTransposed) geom.jacobianInverseTransposed(local);}, localGeometry_);
         }
 
     private: