Add common alias distcalc (j)

[weiji14]

Description of proposed changes

Determine how spherical distances are calculated. See https://docs.generic-mapping-tools.org/6.1/gmt.html#distcalc-full.

• e - Ellipsoidal (or geodesic) mode
• f - Flat Earth mode
• g - Great circle distance [Default]

All spherical distance calculations depend on the current ellipsoid
(PROJ_ELLIPSOID), the definition of the mean radius
(PROJ_MEAN_RADIUS), and the specification of latitude type
(PROJ_AUX_LATITUDE). Geodesic distance calculations is also
controlled by method (PROJ_GEODESIC).

Cherry picked from #546.

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[weiji14]

I'll temporarily disable the Vercel and Travis required checks since the GMT server is down.