Design a logo for PyGMT

[weiji14]

Description of the desired feature

Discussion for a PyGMT logo has been brought up a few times already in meetings and the forum post at https://forum.generic-mapping-tools.org/t/design-for-pygmt-logo/1346. I think it's time for the PyGMT project to have a proper logo in place by the end of the year (for v0.5.0), ideally before the AGU 2021 conference (#1395) in December so we can give out stickers 😁

Logo Design Requirements

As pointed out by Leo at https://forum.generic-mapping-tools.org/t/design-for-pygmt-logo/1346/2, the logo should ideally:

1. Be roughly 1:1 proportion (to make it easier to use)
2. Must be recognizable/understandable at large and small sizes (no small fonts, thin lines, small details)
3. Ideally should work on light and dark backgrounds
4. No use of non-open art work/images

Are you willing to help implement and maintain this feature? Yes, this will be a fun and not very code-heavy project for the ESWN workshop running from 17-19 Aug