Allow WKT2 / Deprecate PROJ

[m-mohr]

Now that we have GDAL3 and WKT2 support baked-in, we should also allow WKT2 in all places where PROJ codes are also allowed.

Some high-level information from ESRI: http://proceedings.esri.com/library/userconf/proc17/tech-workshops/tw_2588-212.pdf

This influences a bunch of processes and some parts of the API.

[m-mohr]

Remove PROJ from API and processes: https://twitter.com/EvenRouault/status/1189515652958212096

[edzer]

Worth checking first that all back-ends (implemented and foreseen/wished) are ready for that change. See also https://lists.osgeo.org/pipermail/gdal-dev/2019-October/050983.html for a more balanced view. This transition was motivated by doing datum transformations properly: do we need datum transformations in openEO?

GDAL3/PROJ6 also bring axis order hell on the table. How does openEO handle that, does it want and/or need to do that?

[m-mohr]

For now, I just marked PROJ as deprecated and added WKT2 / PROJJSON. We'll see later what is getting implemented. So it's still there and I hope the API is future proof.