Improve MeasureGapSize class for tests related with QE Correction

[b1quint]

A recent test for QECorrect started to fail. Chris changed the parameters but the reason it is failing is that the test is not performing as expected. See plot below:

You can see that the data seems fine, but the functions used to adjust the spectra diverge on the second gap due to the existence of a strong absorption line near the edges.