astro/geographiclib: update to 1.51

• New installed default dataset (-igrf12, +igrf13).
• Added option to select precision of real number

Changelog: https://sourceforge.net/p/geographiclib/news/2020/11/geographiclib-151-released-2020-11-22-13/

PR: 251697
Submitted by: Tatsuki Makino <tatsuki_makino@hotmail.com> (maintainer)
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D27525