[!142] change const to constexpr for constant values used as template parameters

Merge branch 'issue/intel_const_constexpr' into 'master'

ref:core/dune-geometry\>

### Summary

Solve an intel-compiler related error, by replacing const by constexpr for
template parameter variables.

See merge request [!142]

  [!142]: gitlab.dune-project.org/core/dune-geometry/merge_requests/142

dune/geometry/virtualrefinement.cc

@@ -522,8 +522,8 @@ namespace Dune
       topologyId &= ~1;
       coerceToId &= ~1;
 
-      const unsigned idSimplex = GeometryTypes::simplex(dimension).id() & ~1;
-      const unsigned idCube = GeometryTypes::cube(dimension).id() & ~1;
+      constexpr unsigned idSimplex = GeometryTypes::simplex(dimension).id() & ~1;
+      constexpr unsigned idCube = GeometryTypes::cube(dimension).id() & ~1;
 
       switch( topologyId )
       {
@@ -573,7 +573,7 @@ namespace Dune
       topologyId &= ~1;
       coerceToId &= ~1;
 
-      const unsigned idSimplex = GeometryTypes::simplex(dimension).id() & ~1;
+      constexpr unsigned idSimplex = GeometryTypes::simplex(dimension).id() & ~1;
 
       if (topologyId == 0 && coerceToId == 0)
         return VirtualRefinementImp< idSimplex, CoordType, idSimplex, dimension>::instance();
@@ -595,10 +595,10 @@ namespace Dune
       topologyId &= ~1;
       coerceToId &= ~1;
 
-      const unsigned idSimplex = GeometryTypes::simplex(dimension).id() & ~1;
-      const unsigned idCube = GeometryTypes::cube(dimension).id() & ~1;
-      const unsigned idPrism = GeometryTypes::prism.id() & ~1;
-      const unsigned idPyramid = GeometryTypes::pyramid.id() & ~1;
+      constexpr unsigned idSimplex = GeometryTypes::simplex(dimension).id() & ~1;
+      constexpr unsigned idCube = GeometryTypes::cube(dimension).id() & ~1;
+      constexpr unsigned idPrism = GeometryTypes::prism.id() & ~1;
+      constexpr unsigned idPyramid = GeometryTypes::pyramid.id() & ~1;
 
       switch( topologyId )
       {