ENH: Add multiple magnetic coordinate systems to `calc_mlat_rings()` function

[KCollins]

Description

calc_mlat_rings calculates the geographic latitudes and longitudes of a circle of points (graticule) for a list of magnetic latitudes. Currently it works only with aacgmv2; this enhancement would entail adding functionality for other coordinate systems as well.

Potential impact

• Is the feature related to an existing problem? -- It would facilitate mapping/comparison of multiple magnetic coordinate systems.
• How critical is this feature to your workflow? -- It's outside the scope of calculating conjugate points, so it isn't critical, but would be useful to have.
• How wide of an impact to you anticipate this enhancement having? -- May use in upcoming IPY5 reports
• Would this break any existing functionality? -- No

Potential solution(s)

• Add "method" string and corresponding case statement
• Write code to calculate graticules for each coordinate system
• Include method string in filename for saving .gpx files
• Validate each method
• Write example notebook to generate map with multiple overlaid coordinate systems

Alternatives

magcoordmap is an alternative, but at the time of writing it only works for apex coordinates.

Additional context

Method tags:

• "aacgm": Calculated using aacgmv2.
• "qdip" : Calculated using apexpy.
• "apex" : Calculated using apexpy.
• "gsm" : Calculated using geopack.
• "auto" : defaults to aacgm.