Add functionality for correct transforms in ax.annotate

[abrammer]

fixes #1720

Rationale

Annotate has been "broken" for quite some time (https://stackoverflow.com/questions/25416600/why-the-annotate-worked-unexpected-here-in-cartopy), and while the work around is somewhat simple, fixing it to be more intuitive also seems fairly straight forward. Also recently raised in the open issue #1720 which spurred me on to open this PR.

I'm not particularly aware of underlying stuff here or if this is the "correct" way to fix it, but it seems like a relatively simple fix.

Implications

ax.annotate will function like most other GeoAxes.methods, but previous work arounds will continue to work.

Examples

added a couple simple tests which exemplify the various annotate transform configurations.

Example annotated maps:

[greglucas]

I think this looks like a reasonable approach.

For the tests, I would suggest combining them all into one with the different cases pointing to different positions, or parameterizing over the transforms with pytest.mark.parametrize(), I'm not sure we need a lot of projections to be tested, more so the points/transforms being moved around.

One thing I don't see is the explicit "data" case? If I'm following correctly, that should be the same as the transform passed into the annotate call, and if there is none, default to the map projection.

It might be good to also test that supplying different transforms for xycoords and textcoords works as expected too, and we aren't accidentally only using one of them.

[abrammer]

@greglucas Thanks for the input. Consolidated the tests and added a couple permutations of providing different CRS transforms to xycoords and textcoords. Parameterized a couple base projections, not sure if that's really needed but it's also pretty simple.

[greglucas]

Looking good. Just a few minor comments. I like the combinations you chose for the tests.

[greglucas]

Looks good to me, just one minor comment left about removing the parametrize now that we're only using one projection (sorry about sending you down that path!).

[abrammer]

yeah, no worries. It was a simple addition and a simple removal.

[greglucas]

Thank you for contributing this, @abrammer! Hope to see you around again.