Let -Jp|P optionally set what angles"w/e" really are

[PaulWessel]

See #5120 for background. Since we cannot tell what the angles are we default to great-circle angles (0-360 or -180/80 depending on FORMAT_GEO_MAP setting, and append the degree symbol) unless users add +kx to select longitude annotations or +ky for latitude annotations. The latter also requires -R to be within -90/90 range.

I labelled this WIP for now since we have to decide if this is a bug fix or a feature that should be included in 6.2.0.

Closes #5120.

[PaulWessel]

This PR yields these failures:

	480 - test/grdinterpolate/slices.sh (Failed)
	481 - test/grdinterpolate/slices_file.sh (Failed)
	645 - test/psbasemap/azim_radial.sh (Failed)
	665 - test/psbasemap/polarapex.sh (Failed)
	666 - test/psbasemap/polarcases.sh (Failed)
	667 - test/psbasemap/polarcircles.sh (Failed)

because they previously all were assumed to be longitudinal crossections for a north pole but in this PR we do not make that assumption. So either (1) add +kx to these examples and then they will pass, or (2) update some scripts/plots to allow for +kx and +ky as well.

[maxrjones]

@PaulWessel, should I review this now or is more work needed first?

[PaulWessel]

Not yet, working on the labels.

[PaulWessel]

This PR is good to be reviewed. It contains these changes:

1. Fixes the bug that angles were always assumed to be longitudes. We do so by offering new modifier +kx|y to select longitudes, latitudes [Default is great circle angles]
2. Adds some checks based on this knowledge (e.g. if +ky then angle range must not exceed ±90).
3. Adds documentation to this effect and updates test plots that falsely used longitude annotations
4. Honors label annotation for the radial direction (depth or radius, for instance) which had been left off earlier since the annotation machinery used is that for geographic maps.
5. Adds three new psbasemap tests to ensure the radial labels are placed correctly for various -JP modifier settings.

All tests pass for me.

[PaulWessel]

Just pinging @joa-quim @meghanrjones @seisman about this finalized PR since it has been awhile.