Stability hotfix

[unalmis]

Fix derivative of abs(sqrt(g)) computation.

• Bug found by Dr. Matt Landreman
• Affected quantities are V_rr(r), D_well, D_Mercier, magnetic well

For documentation, stability compute function results for dshape are below. There were no visible differences for heliotron.

Old:

Notably, the dip near the boundary is gone.

[unalmis]

I suggest merging this and #251 now. We can tighten stability function tolerances in #261 as that is one of its goals. Tolerances are not tightened here because this branch lacks certain grid fixes.

[codecov]

Codecov Report

Base: 90.99% // Head: 90.99% // No change to project coverage 👍

Coverage data is based on head (fb92bc3) compared to base (9a065a7).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #266   +/-   ##
=======================================
  Coverage   90.99%   90.99%           
=======================================
  Files          56       56           
  Lines       11840    11840           
=======================================
  Hits        10774    10774           
  Misses       1066     1066           

Impacted Files	Coverage Δ
desc/equilibrium.py	89.32% <ø> (ø)
desc/perturbations.py	84.87% <ø> (ø)
desc/compute/_core.py	97.46% <100.00%> (ø)
desc/compute/_stability.py	100.00% <100.00%> (ø)

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[f0uriest]

Looks fine.

[dpanici]

Looks good