Add constants.py

[moeyensj]

We should add a constants class that stores all relevant constants as opposed to relying on those provided by astropy.

[mjuric]

Are the ones in astropy incoorrect (or tend to change)?

Not opposed, just curious...

[moeyensj]

I am not sure if they are correct or incorrect, but they are different from the ones provided in the JPL ephemeris files (which is where oorb draws its constants from).

For example:

from astropy import constants as c
from astropy import units as u

desired_units = (u.AU**3 / u.d**2)
MU_astropy = (c.G * u.M_sun).to(desired_units).value
MU_de430 = 0.295912208285591100e-3 # in units of AU^3 / d^2 from page 49 in https://ipnpr.jpl.nasa.gov/progress_report/42-196/196C.pdf

print(1 - MU_astropy/MU_de430)
3.1601721239837843e-10

[mjuric]

👍.

(n.b., I was curious about this, so I checked what they do: astropy constants do change from version to version, though there is a way to fix which version a code should use https://docs.astropy.org/en/stable/constants/, https://github.com/astropy/astropy/blob/master/astropy/constants/.)

[moeyensj]

Completed with #17