BUG: setting GeoAxes extent

[chrishavlin]

This changes how the extent for GeoAxes is handled -- this version calls set_extent only if the data is not global and always passes the extent kwarg to imshow. Seemed to work well locally, but we'll see how CI does...

I may broaden the scope of this PR to include changing the default projection to set the central_longitude kwarg from the plot arguments when available, but if that proves to be too complex I'll just keep this PR to the extent modification. EDIT: Will look into this for a follow up PR.

[chrishavlin]

So in the latest batch of commits, among other things, I adjusted the test_geo_projections tests to use a linear instead of log scale for the colorbar scaling to avoid the matplotlib issue. I'm not entirely sure why it was failing now, but it was related to similar failures we've had with lognorm and certain data ranges. Since that should separately be fixed by #3849 and #3986, I just switched the tests here to use a linear scale. Could switch back at a later date, but I don't see a reason to not leave it using the linear scaling.

[neutrinoceros]

Two quick comments, otherwise LGTM

[chrishavlin]

oops, just pushed to this branch accidentally... ignore the latest changes, will fix....

[chrishavlin]

ok. fixed the accidental push, updated the geoaxes check. and I want to add one more test then this will be good I think!

[neutrinoceros]

I forgot to check that sys was already imported in my last suggestion. It wasn't.

[neutrinoceros]

I fixed the 2 issues (one purely stylistic, reported by flake8) with my latest suggestion.
I'll review again after lunch when CI is complete

[neutrinoceros]

I think it's good to go. Before I merge, does this completely fix #4039 (which isn't currently linked) ?

[chrishavlin]

Yes, this does fix the bug there. The other part of the discussion in #4039 (setting the central_longitude of the default projection when possible) is more of an enhancement that will take some extra work, I'll open up a new issue to track it.