Rename RT prebasis from 'rt' to 'raviartThomas'

Trying to get towards a meaningful naming scheme.  Here int particular,
'rt' is not a good name, because it could mean both 'Raviart-Thomas'
and 'Rannacher-Turek'.

dune/functions/functionspacebases/raviartthomasbasis.hh

@@ -423,7 +423,7 @@ public:
  * \tparam basic_type Basic geometry type
  */
 template<std::size_t k, GeometryType::BasicType basic_type, class size_type=std::size_t>
-auto rt()
+auto raviartThomas()
 {
   return Imp::RaviartThomasPreBasisFactory<k, basic_type, size_type>();
 }

dune/functions/functionspacebases/test/raviartthomasbasistest.cc

@@ -43,7 +43,7 @@ int main (int argc, char* argv[])
   // check RaviartThomasBasis created using basis builder mechanism
   {
     using namespace Functions::BasisBuilder;
-    auto basis = makeBasis(grid.leafGridView(), rt<0, GeometryType::BasicType::cube>());
+    auto basis = makeBasis(grid.leafGridView(), raviartThomas<0, GeometryType::BasicType::cube>());
     test.subTest(checkBasis(basis));
   }
 

examples/poisson-mfem.cc

@@ -361,7 +361,7 @@ int main (int argc, char *argv[])
   auto basis = makeBasis(
     gridView,
     composite(
-      rt<k, GeometryType::BasicType::cube>(),
+      raviartThomas<k, GeometryType::BasicType::cube>(),
       pq<k>()
     ));