Add a simulation unit system class

[adrn]

Describe your changes

This is a simple implementation, but I found it sitting in a notebook so I decided to just clean it up and add it here. Does this look like it would satisfy your use cases with pyexp @jngaravitoc?

I need to add tests and some documentation...

Checklist

• Did you add tests?
• Did you add documentation for your changes?
• Did you reference any relevant issues?
• Did you add a changelog entry? (see CHANGES.rst)
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[adrn]

@jngaravitoc I'm not sure what EXP gives you or assumes for units, but let's say you have to set a mass scale, time scale, and G=1. If you have output from an EXP sim as positions exp_xyz and exp_vxyz, you could do this:

import astropy.units as u
usys = SimulationUnitSystem(mass=1e6 * u.Msun, time=100*u.Myr)
(exp_xyz * usys["length"]).to(u.kpc)
(exp_vxyz * usys["velocity"]).to(u.km/u.s)

[jngaravitoc]

Thanks @adrn! I think for EXP when analyzing external simulations what we will do is: usys = SimulationUnitSystem(mass=1 * u.Msun, length=1*u.kpc, G=1)

[The9Cat]

exp assumes nothing about units, other than G=1. In other words, the user provides positions and velocities and that determines the time units. This seemed like a clean choice at the time (EXP actually predates stuff like Gadget).

Anyway, Adrian's solution is tré cool!