When using different line widths during a single run, ensure that the fluxes and other quantities are always computed correctly

Ensure that the computation of the continuum level is correct when determining the equivalent width, in particular when the line width is very narrow.

The formula from Sawicki et al. (2012) used to compute the χ² in the presence of upper limits was not correct. This led the χ² to depend directly on the absolute value of the upper limit. The formula has been rederived and corrected.

Fν was computed by calculating Fλ and then converting to Fν, which led to typical differences in fluxes of typically 1-2% and a bit more for a handful of pathological filters. Now Fν is computed directly and a bit faster. At the same time rather than computing an effective wavelength (which was more a mean wavelength), we compute the pivot wavelength, which is more appropriate when converting between Fν and Fλ.

When computing the attenuation curve, the bump is now added so that its relative strength does not depend on δ.

For more reliability we now save the chi² by blocks. Otherwise it could lead to some crashes when there was more than 1 redshift.