Fix calibration when 3b is totally deactivated

[mraspaud]

Fully deactivated 3b channel can crash the calibration. This PR fixes this.

This fixes #78 and fixes #22 in principle.

[codecov]

Codecov Report

Merging #117 (9f30e02) into main (28265cd) will increase coverage by 1.23%.
The diff coverage is 97.05%.

@@            Coverage Diff             @@
##             main     #117      +/-   ##
==========================================
+ Coverage   73.94%   75.18%   +1.23%     
==========================================
  Files          30       30              
  Lines        2698     2732      +34     
==========================================
+ Hits         1995     2054      +59     
+ Misses        703      678      -25     

Impacted Files	Coverage Δ
pygac/pod_reader.py	63.63% <50.00%> (-0.23%)	⬇️
pygac/klm_reader.py	68.11% <100.00%> (+8.71%)	⬆️
pygac/reader.py	81.77% <100.00%> (+2.21%)	⬆️
pygac/tests/test_klm.py	100.00% <100.00%> (+4.76%)	⬆️
pygac/tests/test_reader.py	98.35% <100.00%> (+0.01%)	⬆️
pygac/calibration.py	94.73% <0.00%> (+0.52%)	⬆️
pygac/lac_klm.py	82.35% <0.00%> (+17.64%)	⬆️

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[sfinkens]

LGTM! And thanks for cleaning up the tests