Automatic conversion of ``std::shared_ptr<T>`` is not possible when ``T`` is
not directly registered with ``py::class_<T>`` (e.g. ``std::shared_ptr<int>``
or ``std::shared_ptr<std::vector<T>>`` are not automatically convertible).

In this case all we need to do is get rid of an overload that was
not being used. But I also made the existing overload more efficient
by using std::move (as suggested by Pim Schellart) and simplified
the associated code comments.

declareGetImage should wrap Background::getImage
but not BackgroundMI::getImage (since that is inherited).
pybind11 2.2 rejected the old code.

Still to fix:
    from .image import *
E   ImportError: arg(): could not convert default argument 'planeDefs: std::__1::map<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, int, std::__1::less<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const, int> > >' in method '<class 'lsst.afw.image.image.image.MaskX'>.__init__' into a Python object (type not registered yet?)

which appears to be this bit of code:
    cls.def(py::init<unsigned int, unsigned int, typename Mask<MaskPixelT>::MaskPlaneDict const &>(),
            "width"_a, "height"_a, "planeDefs"_a = typename Mask<MaskPixelT>::MaskPlaneDict());

note that MaskPlaneDict is defined as follows in Mask.h:

    typedef std::map<std::string, int> MaskPlaneDict;

Including coord, cameraGeom, display, detection, fits,
geom, math, math.detail and also the wrapper in test/.
Use addUnderscore=False to SConscript.
Use <x>Continued.py and subdirectories where appropriate.
Use @continueClass where appropriate.
Delete unused wrappers, which had no content and were not built.
Change the way the code pulled the contents of lsst.afw.math.detail
into lsst.afw.math to avoid the `convolve` module
(formerly `_convolve`) shadowing the `convolve` function.

Make it accept a vector of vectors instead of a vector of
shared_ptr to vectors,
and return a vector instead of a shared pointer to a vector.

pybind11 2.2 appears to be much pickier than 2.1 about requiring
wrapper modules to import what they need.