issue with symmetric grids

[unalmis]

Fixes issues with symmetric grids.

• On LinearGrid, specifying nodes at coordinates $\theta \in$ [0, $\pi$] is now sufficient when sym=True.
• When sym=True, nodes at coordinates $\theta$ mod $\pi = 0$ should not have their $d\theta$ scaled up.
• LinearGrid can now handle non-uniformly spaced nodes if the user passes them when sym=True.
• Corrects grid.weights on ConcentricGrid with sym=True.
  • Resolving example equilibria and tightening those test tolerances to be done in different PR.
• More tests.

[codecov]

Codecov Report

Patch coverage: 96.72% and no project coverage change.

Comparison is base (6a06623) 93.84% compared to head (166e30d) 93.84%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #451   +/-   ##
=======================================
  Coverage   93.84%   93.84%           
=======================================
  Files          72       72           
  Lines       15008    15019   +11     
=======================================
+ Hits        14084    14095   +11     
  Misses        924      924           

Impacted Files	Coverage Δ
desc/grid.py	98.20% <96.72%> (+0.05%)	⬆️

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[unalmis]

If some of the grid spacing tests are a chore to review, it might be easier to just check results directly. For convenience:

import sys
import os
sys.path.insert(0, os.path.abspath("."))
# sys.path.append(os.path.abspath("../../"))
from desc.grid import *
from desc.plotting import plot_grid
import numpy as np

theta = np.linspace() or integer
lg = LinearGrid(rho=, theta=, zeta=, sym=)
print(lg.spacing[:, 1])
plot_grid(lg)