fix isssue that prevented to update the bins in plot_migration_matrix

[vuillaut]

fixes #203

[codecov]

Codecov Report

Attention: 105 lines in your changes are missing coverage. Please review.

Comparison is base (eafbe37) 63.15% compared to head (a70cc66) 63.23%.
Report is 38 commits behind head on master.

Files	Patch %	Lines
ctaplot/gammaboard/gammaboard.py	2.94%	99 Missing ⚠️
ctaplot/ana/ana.py	86.66%	4 Missing ⚠️
ctaplot/gammaboard/reader.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
+ Coverage   63.15%   63.23%   +0.07%     
==========================================
  Files          21       21              
  Lines        2717     2731      +14     
==========================================
+ Hits         1716     1727      +11     
- Misses       1001     1004       +3     

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