Fix up options for plotting X-points in plot overlays

[eldond]

• Add missing item to docstring
• Add control of measured X-point plot marker in shape target overlay
  • old efitviewer includes measured X-point here, so that's why it's there
• Add measured X-point as an option in equilibrium CX

[codecov[bot]]

Codecov Report

Merging #86 into master will increase coverage by 0.06%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   75.77%   75.84%   +0.06%     
==========================================
  Files          24       24              
  Lines        6969     6989      +20     
==========================================
+ Hits         5281     5301      +20     
  Misses       1688     1688              

Impacted Files	Coverage Δ
omas/omas_setup.py	84.81% <0.00%> (ø)	:arrow_up:
omas/omas_plot.py	74.03% <0.00%> (+0.31%)	:arrow_up:

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