[draft] Allow equality comparison of TransformedRangeIterator classes with same base iterator

So far comparision was only provided for identical TransformedRangeIterator type. This is generalized to allow equality/inequlity comparison of different types as long as the base iterator is the same. This is reasonable, since comparisons anyway rely on the base type only. The patch is needed if we want to allow to generate TransformedRangeIterators only with lambdas, since each lambda has a different type.

The latter functionality is tested and demonstrated in the respective test. We may later want to provide a public factory method for creating such iterators (which should probably stay in Dune::Impl:: themselves).

dune/common/test/rangeutilitiestest.cc

@@ -8,6 +8,7 @@
 #include <vector>
 #include <numeric>
 #include <type_traits>
+#include <optional>
 
 #include <dune/common/hybridutilities.hh>
 #include <dune/common/iteratorrange.hh>
@@ -289,6 +290,18 @@ auto testTransformedRangeView()
     suite.check(checkSameRange(rc, std::vector{1, 2, 3}))
       << "accessing mutable range via const reference failed";
   }
+  {
+    auto transformedIterator = [](auto&& it, auto&& f) {
+      using It = std::decay_t<decltype(it)>;
+      using F = std::decay_t<decltype(f)>;
+      using TIt = Dune::Impl::TransformedRangeIterator<It, std::optional<F>, Dune::ValueTransformationTag>;
+      return TIt(std::forward<decltype(it)>(it), std::forward<decltype(f)>(f));
+    };
+    auto it = transformedIterator(Dune::range(0,5).begin(), [](auto x) { return 2*x; });
+    auto end = transformedIterator(Dune::range(0,4).end(), [](auto x) {return 0;});
+    for(; it!=end; ++it)
+      std::cout << *it << std::endl;
+  }
   return suite;
 }