Add Equal Earth projection

[berniejenny]

Equal Earth is a projection inspired by the Robinson projection, but unlike the Robinson projection retains the relative size of areas. The projection is described in this publication:

Bojan Savric, Tom Patterson & Bernhard Jenny (2018).
The Equal Earth map projection.
International Journal of Geographical Information Science.
http://doi.org/10.1080/13658816.2018.1504949

Java code is available here: https://github.com/OSUCartography/JMapProjLib/blob/master/src/com/jhlabs/map/proj/EqualEarthProjection.java

[kbevers]

Interesting paper! And the projection is definitely a worthy candidate for inclusion in PROJ,I think. I'd be happy to mentor a pull request implementing and documenting the new projection. If you are quick it can be included in PROJ v. 5.2.0 scheduled for release September 1st.

[jdugge]

@kbevers I've given this a shot, using @mbloch's and @berniejenny's Java / JavaScript code as a reference.

Could you take a look and tell me what's next (tests, documentation)?

[rouault]

tell me what's next (tests, documentation)?

Tests can be added to test/gie/builtins.gie (not completely sure if that's the most appropriate one), and docs in docs/source/operations/projections/ (new file + reference to it in docs/source/operations/projections/index.rst)

[thareUSGS]

I look forward to using the Equal Earth projection on Mars - doh. 🌝