Add Lambert Equal Area Projection

[lbdreyer]

This adds the lambert equal area projection.

Guidance with the x and y limits would be appreciated. The numbers I have used are temporary and just made the figure look about right.

I am also unclear as to whether there should be extra keywords e.g. scale factor.

(I have an implementation in Iris ready once this has merged.)

[rhattersley]

Thanks @lbdreyer - this is a really good submission.

[rhattersley]

This needs to include "Azimuthal" as there is also a cylindrical alternative.

Are you sure this is the equal-area projection you are after? (It probably is.)

[rhattersley]

I am also unclear as to whether there should be extra keywords e.g. scale factor.

From a quick search around it looks OK as it is.

[rhattersley]

The theoretical X axis limits are ±a * 2 + false_easting. But as with other projections it's best to reduce those limits a little to avoid problems at the boundary, e.g. (-a * 1.99999 + false_easting, a * 1.99999 + false_easting).

[pelson]

@lbdreyer - this looks good.
I just noticed you aren't on https://github.com/SciTools/cartopy/blob/master/docs/source/contributors.rst. Did you want to add yourself? (feel free to do it in this PR)

I've also tested the docs when make_projection.py is run and it looks fine, so please go ahead and run that (it will update source/crs/projections.rst) as part of this PR.

[QuLogic]

Missing a period here.

[rhattersley]

👍 I'm happy to merge after you've squashed the commits. (Currently: bf3ad97)

[rhattersley]

Thanks everyone 😄

[pelson]

👍 - awesome. Thanks @lbdreyer!

[rhattersley]

Now we just need to get it released... 😉