workaround for communicators in blocked bases, use SequentialCommunication if not flat MultiIndex

This workaround is implemented to at least allow that blocked bases compile with ParallelGlobalBasis. By default, if the basis is blocked, i.e. the MultiIndex is not a flat MultiIndex, the basis uses a SequentialCommunication instead of a backend communication object.

src/amdis/functions/ParallelGlobalBasis.hpp

@@ -23,12 +23,13 @@
 
 #include <dune/typetree/treepath.hh>
 
-#include <amdis/common/Concepts.hpp>
-#include <amdis/common/TypeTraits.hpp>
-#include <amdis/linearalgebra/Traits.hpp>
 #include <amdis/AdaptiveGrid.hpp>
 #include <amdis/Observer.hpp>
 #include <amdis/Output.hpp>
+#include <amdis/common/Concepts.hpp>
+#include <amdis/common/TypeTraits.hpp>
+#include <amdis/linearalgebra/Traits.hpp>
+#include <amdis/typetree/MultiIndex.hpp>
 
 #if DUNE_VERSION_LT(DUNE_FUNCTIONS,2,7)
 #include <dune/functions/functionspacebases/defaultlocalindexset.hh>
@@ -65,7 +66,9 @@ namespace AMDiS
     using LocalView = Dune::Functions::DefaultLocalView<Self>;
 
     /// Type of the communicator
-    using Comm = typename BackendTraits<Self>::Comm;
+    using Comm = std::conditional_t<Traits::IsFlatIndex<typename Super::MultiIndex>::value,
+      typename BackendTraits<Self>::Comm,
+      SequentialCommunication>;
 
     struct DummyImpl {};
     using ADH = Dune::AdaptDataHandle<Grid, DummyImpl>;