Add new method 'size' to class MultiTypeBlockVector, which returns the vector size

The class already contains a method 'count', which returns the same value.
However, all our other vector types use 'size' for the vector size, not
'count'.

Additionally, the method is static and DUNE_CONSTEXPR, which is sometimes
helpful.

dune/istl/multitypeblockvector.hh

@@ -256,6 +256,12 @@ namespace Dune {
 
     typedef typename T1::field_type field_type;
 
+    /** \brief Return the number of vector entries */
+    static DUNE_CONSTEXPR std::size_t size()
+    {
+      return mpl::size<type>::value;
+    }
+
     /**
      * number of elements
      */

dune/istl/test/multitypeblockvectortest.cc

@@ -37,6 +37,9 @@ int main(int argc, char** argv) try
 
   // test method 'count'
   std::cout << "multi vector has " << multiVector.count() << " first level blocks" << std::endl;
+
+  static_assert(multiVector.size()==2, "Method MultiTypeBlockVector::size() returned wrong value!");
+
   if (multiVector.count() != 2)
     DUNE_THROW(Exception, "Method MultiTypeBlockVector::count returned wrong value!");