[PROJ] Derived Vertical CRS

[Even Rouault]

> Regarding the WKT issue, I've raised this to the OGC CRS SWG list as 
> the grammar of WKT doesn't allow currently a unitless parameter, even 
> if the EPSG Vertical Offset and Slope method requires it. I'd suggest 
> using something like UNIT["none",1] instead of 
> ANGLEUNIT["arc-second",4.8481368111E-06], until that's cleared up
>
Got confirmation that requiring the unit was not intended and will be 
amended in the next revision of the WKT CRS specification.

I've corrected the WKT parser to allow unit-less PARAMETER for 
coordinate operations: https://github.com/OSGeo/PROJ/pull/3271

Even

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