SRS barn raising: 2nd report

This is the second progress report of the GDAL SRS barn effort.

Since the previous report, a number of topics have been addressed:

- extension of the class hierarchy to implement BoundCRS (the generalization of the WKT1 TOWGS84 concept. This concept only exists in WKT 2 and has not been modeled in ISO-19111, so I went on my own modelling), TimeCRS, DerivedGeodeticCRS

- implementation of the exportToPROJ() method for CRS and related objects, and CoordinateOperation

- addition of all documentation needed at class and method level so that Doxygen passes without warnings

- implementation of CoordinateOperation::createOperation() method that can instanciate a transformation between two CRSs. For now, it does not use yet the operations of the EPSG database, but it can already take into account the hints of BoundCRS.

- implementation of a number of Transformations: geocentric translation, position vector transformation, coordinate frame rotation, NTv2,  GravityRelatedHeightToGeographic3D, VERTCON.

- start of mapping all GDAL currently supported projection methods. For now: UTM, Transverse Mercator, Transerve Mercator South Oriented, Two Point Equidistant, Tunisia Mapping Grid, Albers Conic Equal Area, Lambert Conic Conformal 1SP, New Zealand Map Grid. Several tens are obviously still missing.

- addition of a isEquivalentTo() method to compare the various objects.

- and of course, extensive unit testing of all the above work.

The result of this continued work can be followed in this pull request.

As a related effort, I've coordinated with the OGC CRS Working Group to provide my comments on the upcoming ISO:19168 / WKTv2 2018 revision.