Don't flatten Potential instances

[nstarman]

first check its a Potential

Signed-off-by: Nathaniel Starkman (@nstarman) nstarkman@protonmail.com

[nstarman]

I also got CompositePotential to completely work. It operates and looks like a list. should I push in different PR?
It also works when nested, just like the list of potentials.

[nstarman]

See https://github.com/nstarman/galpy/tree/composite_potentials

[jobovy]

Thanks, this is a very straightforward way to fix the issue you're having while not affecting any of the other code!

Let's hold off for now on the CompositePotential, I need to think more about how that would mesh with the rest of the code.

With travis slow these days, it will be a while until the tests all run...

[codecov]

Codecov Report

Merging #436 (f7327cd) into master (5afb023) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #436   +/-   ##
=======================================
  Coverage   99.82%   99.82%           
=======================================
  Files         176      176           
  Lines       25332    25380   +48     
=======================================
+ Hits        25287    25335   +48     
  Misses         45       45           

Impacted Files	Coverage Δ
galpy/potential/Potential.py	100.00% <100.00%> (ø)
galpy/util/coords.py	100.00% <0.00%> (ø)

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