[bugfix] Prohibit copy constructor of DenseVector

DenseVector is the base class of a CRTP. This is kind of an abstract
class that works only if it can be casted into a derived type. Copying
the base class is pointless.

This patch marks the copy constructor of DenseVector private (not even
implementing it), so that it cannot be called anymore. Once we have a
DUNE_DELETE macro, we should use it, here.

At the same time, the default constructor is marked protected, so that only
derived classes can instantiate a DenseVector. Notice that this does not
prevent the user from building a derivced class which does not pass
itself as implementation to DenseVector. It is (to my knowledge) not possible
to make CRTPs completely fail-safe.