Add legend to Gammaboard resolution curves

[mikael10j]

solves #181

[codecov]

Codecov Report

Merging #187 (8b7140e) into master (eafbe37) will increase coverage by 0.03%.
The diff coverage is 31.81%.

@@            Coverage Diff             @@
##           master     #187      +/-   ##
==========================================
+ Coverage   63.15%   63.19%   +0.03%     
==========================================
  Files          21       21              
  Lines        2717     2733      +16     
==========================================
+ Hits         1716     1727      +11     
- Misses       1001     1006       +5     

Impacted Files	Coverage Δ
ctaplot/gammaboard/reader.py	0.00% <0.00%> (ø)
ctaplot/plots/style.py	100.00% <ø> (ø)
ctaplot/gammaboard/gammaboard.py	11.81% <2.32%> (-0.10%)	⬇️
ctaplot/ana/ana.py	88.20% <85.71%> (-0.03%)	⬇️
ctaplot/__init__.py	100.00% <100.00%> (ø)
ctaplot/ana/tests/test_ana.py	100.00% <100.00%> (ø)
ctaplot/plots/plots.py	78.46% <100.00%> (ø)
ctaplot/plots/tests/test_plots.py	98.73% <100.00%> (+0.07%)	⬆️

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