Unclear error when the time cut eliminates all of the signal for a given minf

Created by: SylvainMarsat

When the time cut (DeltatL_cut) is such that all the signal is below the given minf, rather uninformative error at the level of the C code:

Invalid input value error in BuildFrequencyGrid (lisabeta/tools/tools.c:280): Input freq limits do not verify f_min < f_max.

A temporary fix is to adjust minf to be low enough so that this situation cannot happen (easier done with analytic noise, more annoying with numeric noise).