Time dependent potential origins - galax tutorial

[dc-broo3]

I've written up a short-ish GitHub gist (https://gist.github.com/dc-broo3/5bab8c566d747f0b4cde3b58e1295cad) for the time-dependent origin of potentials in galax. Not sure if this is useful in this format, but happy for it to be included in the galax documentation as a tutorial when that comes around!

There are a few extra files that go along with that script for it to run (e.g., the MW centre from Nico's mwlmc sims). If you need them I can send across when the time comes!