[PROJ] Deformation model implementation

[Even Rouault]

Hi,

Over the course of past years, we discussed here or in
https://github.com/OSGeo/PROJ/issues/1001 how to extend PROJ to deal with deformation 
models able to cope with 3D time-dependent deformations, and particularly, multiple ources 
of deformations with different time constants, such as secular velocity models for GIA 
(glacial isostatic adjustment), or complex deformations due tectonic events such as 
earthquakes, which may include both coseismic deformation (a step function) as well an 
ongoing post-seismic deformation signal.

Chris Crook wrote a specification of an encoding for such a deformation model [1], consisting 
of a main JSON file describing general metadata and parameters of the model, and listing 
different spatial components with their associated time function (velocity, step/reverse step, 
constant, exponential, piece-wise). The grids of each spatial component use the Geodetic 
TIFF grids (GTG) profile of GeoTIFF ([2]) that is available since PROJ 7.0.

The PROJ implementation for this deformation model specification is now available for 
review in the pull request
https://github.com/OSGeo/PROJ/pull/2206 . Chris has tested it on the NZGD200 deformation 
model.

Even

[1] 
https://github.com/linz/deformation-model-format/blob/master/doc/
JsonGeotiffDeformationModelFormat_20200501.pdf
[2] https://proj.org/specifications/geodetictiffgrids.html

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