Check that the range of "x-values" enclose the peak

[celinedurniak]

Check that min and max values of "x" surround the center of the peak ("center")
Check that these values are not NaN...

[gonzalezma]

The requirement of having x_min < peak_center (=0 in most cases) < x_max depends on the limitations that we want to impose to the use of the models. In principle, nothing forbids to compute the values of a model in a region that does not include the peak center or even in a single point, even if in practice I cannot see right now a big utility to this. To note also that if one adds inelastic models to the library, then the definition of the "peak" region is less clear.