Basic user interface in hilbert.py set.

Now the Basic user interface and code structure in hilbert.py is set.
The code is refactored in a way that the QuantumNumber and
QuantumNumberSets can be opaque to the user and need not to be
constructed explicitly by the user.

The actual_value attribute of QuantumNumber is removed. Now the purpose
is recommended to be fulfilled by constructing an operator for that
purpose.