Main change is that features take reference to containing object instead
of pointer, as the feature must always belong to some object. Feature
groups now return references to features and features return reference
to containing object, as these cannot be null.
Passing `*this` to AbstractFeature (and Camera[23]D) constructor might
now clash with deleted copy constructor, added templated constructor to
catch and resolve these ambiguous cases.
Makes some cases less consistent (and some convenience shortcuts
impossible), but goes well with the attitude "don't use pointer when it
can't be null".