Current work: datum shifts by geocentric translations

[Martin Desruisseaux <ma...@geomatys.com>]

Hello all

There is an update on recent work and work in progress:

  * The framework for map projections is in place. Note that it does not
    yet include:
      o Datum shifts (this is the work currently in progress),
      o Automatic inference of the projections between two Coordinate
        Reference Systems (this work will be done after the port of EPSG
        database).
  * Rémi recently added the Oblique Stereographic projection. More
    projections will be added progressively as time allow, but this is
    not the main focus for now.
  * Completed GML support in classes that we left without GML in SIS 0.6
    release.
  * Added support of xlink:href="#foo" in XML documents (both metadata
    and GML). This reduce a lot the verbosity of some documents.

The current work is a port of the code performing Geographic/Geocentric
conversions, then a translation, rotation and scale in the geocentric
space. This is one of the methods used for performing datum shift (i.e.
changing the ellipsoid that approximate the shape of the Earth).
Compared to the code that existed previously in Geotk, we are doing a
redesign made possible by the fact that the EPSG database is now more
accurate about the domain (geocentric, geographic 2D or geographic 3D)
in which the operation is performed than when the original Java code was
written. I think that leveraging this information will help us to avoid
some complicated code in Geotk (not yet ported to SIS, would hopefully
not be needed anymore).

After the port of Geographic/Geocentric conversions and Abridged
Molodensky transformation, I plan to start (at last) the port of the
EPSG database support.

    Martin