[Proj] Understanding 2D Helmert

[Even Rouault]

> It would probably also be possible to invent a simple text based file format
> for storing the triangles and their related parameters. This would be sort
> of equivalent to a grid file that can be linked to in the database. Is the
> current setup geared for something like that, or is it strictly reserved
> for grid files?

There are 2 possibilities I can see with the current state of the code. Either 
use the table that is dedicated to grid transform and invent a method name for 
that method, and add the code that translates this into a PROJ string. This 
has the advantage that the method that the CoordinateOperation::gridsNeeded() 
can be used and users can know that the transform depends on a external 
resource they must possibly grab from somewhere. Or just use the capability of 
putting an arbitrary PROJ string to describe a transformation between 2 CRSs 
codes (blackbox approach).

Even

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