Add Einasto potential

[johnwez1]

Added the Einasto density profile using many of results and conventions in https://arxiv.org/pdf/1202.5242

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (7cfe3bf) to head (e0a7805).

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #760    +/-   ##
========================================
  Coverage   99.91%   99.91%            
========================================
  Files         206      208     +2     
  Lines       30060    30196   +136     
  Branches      620      619     -1     
========================================
+ Hits        30033    30169   +136     
  Misses         27       27            

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[jobovy]

Thanks for this, this looks great! I'll need a bit of time to do a full review, but I did a quick review and everything looks pretty good. Two quick comments:

• A bunch of tests fail because of an unrelated issue with numexpr that I'm fixing in Change numexpr unparseable error test now that logical operations are supported in numexpr #761. Once that's merged, you could rebase to the updated main and the tests should then fully run
• I understand the simplicity of the r/h form of the Einasto profile, but usually I think people use the form from equations (3) or (4) from the paper you link to, so it might be useful to support the alternative rs or rm2 (r_{-2}) forms of the profile as well. The rm2 one is easy to translate to h with equation (7), but even the rs one should be straightforward with a numerical solution to equation (15) or just using equation (16) (or use equation (16) as an initial guess for solving equation (15) numerically...).
• I think you meant to link to the paper in the docs?

[johnwez1]

Thanks Jo. I'll add the other options in.

[jobovy]

Hi! Just a quick note that I'll be releasing a new version of galpy later this week, so if you wanted to get this into that, now's the time. But no worries if not, there's a release every four months, so it can also wait until the next one!

[jobovy]

Thanks @johnwez1 ! Because it's a bit tedious to cover all of the test bases and because the pre-commit failure was a bit annoying to fix, I've pushed a few commits to add the final tests, tweak the code a bit, and fix the pre-commit issue. I'm also going to rebase this to clean up the history, so we can keep a bit of history rather than squashing everything. But then it should be good to go, thanks again for your contribution, this is great!