mongoBackend "redirect" to context provideres for queryContext and updateContext operations

This PR comprises the part of issue #499 regarding mongoBackend.
See 'operator[]' of ContextRegistrationResponseVector.

You can now use (if you want to):

    crrV[0]->contextRegistration.providingApplication.get()

This operator is implemented for:
- ContextRegistrationResponseVector,
- EntityIdVector,
- ContextElementResponseVector and
- ContextAttributeVector