ref: 8a3a65c8c0340253beaba1c2e5687a000fa74789 gio-example/galaxy d---------
galaxy: adapt donated code to fit gio visualization

This commit adapts the donated galaxy simulation code to work
with the front-end visualization originally posted here:


With this change, the entire galaxy example codebase in now under the
same licenses, and visualization code has been better documented.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>
galaxy: add simulation engine for galaxy example

This is essentially the code for the Gonum Barnes-Hut example code and
is donated to the Gio project.

Signed-off-by: Dan Kortschak <dan@kortschak.io>