[PROJ] Vertical CRS transformations and vertical grid shift

[Even Rouault]

> I'm not sure to have fully understood how the bilinear interpolation is
> being used in this process.

Hopefully as (loosely) described in http://www.epsg.org/Portals/0/373-07-2.pdf , "4.10.2 
Vertical Offset by Interpolation of Gridded Data"

Implementation starting at:
https://github.com/OSGeo/PROJ/blob/master/src/grids.cpp#L3178

> Could you please do an example?

Left as an exercice to the reader: peek a point on a grid intersection, look at the value in the 
grid file and verify that PROJ exactly offsets elevation by this value. Now takes a point at the 
middle of 4 grid nodes, compute the mean, and check that PROJ applies this interpolated 
value.

Even

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