Add `terminator_lonlat` and `limb_lonlat` methods to get terminator and limb longitude/latitude coordinate arrays

[ortk95]

Added terminator_lonlat and limb_lonlat to generate arrays of longitude and latitude coordinates for the terminator and limb respectively. These are equivalents to the already-existing terminator_radec and limb_radec methods that generate RA/Dec coordinate arrays. The only difference is that the default value of only_visible is True for terminator_radec, but False for terminator_lonlat, as the lon/lat arrays are less directly linked to the observer's perspective.

These new methods are mainly useful when plotting the limb and terminator lines on mapped data.

Closes #376

Pull request checklist

• [x] Add a clear description of the change
• [x] Add any new tests needed
• [x] Run spell check on new text visible to user (documentation, GUI etc.)
• [x] Check any changes to requirements.txt are reflected in setup.py and conda-forge feedstock
• [x] Check code passes CI checks (run run_ci.sh or check GitHub Actions)

See CONTRIBUTING.md for more details.

[coveralls]

Pull Request Test Coverage Report for Build 9713849711

Details

• 13 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 99.612%

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Totals
Change from base Build 9713261710:	0.001%
Covered Lines:	3116
Relevant Lines:	3128

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