[Proj] C++ coding practices w.r.t object ownership

[Martin Desruisseaux]

Hello Even

Thanks for this first draft, it looks promising!

One quick comment: it may be worth to move CI_Citation, EX_Extent and
MD_Identifier, currently in "common", to their own "metadata" package.
The reason is that even if they are currently the main ISO 19115
structures of interest for Proj, in the future GDAL may be interested in
more ISO 19115 metadata structures (e.g. GeographicBoundingBox,
SampleDimension, CoverageDescription, etc.).

    Martin