Remove forbidden downcast

This removes several constructors and assignments from base class in the class hierarchy around base_array. These removed methods are all implemented using a downcast which may result in undefined behaviour. Since they are not used anywhere in the core modules we can maybe remove them. According to @markus.blatt these base classes are all meant to be internal.

This may cause problems if user code explicitly uses the raw base classes and these methods. However, such user code would only be valid if the passed base class reference is actually an upcasted derived class which is very unlikely.

dune/istl/test/basearraytest.cc

@@ -11,9 +11,6 @@ int main()
   base_array<double> v1(10);
   base_array<double> v2 = v1;
 
-  // Test constructor from base_array_unmanaged
-  base_array<double> v3 = *(static_cast<base_array_unmanaged<double>*>(&v1));
-
   v1.resize(20);
 
   v1 = v2;