foucalc

• It would be very helpful if the foucalc help is updated with actual examples how to use it and a throughout description what it does (including the normalization convention of the FFT in ISIS, which is 1/sqrt(N).
• Adjust warning of "gaps in LC": check whether this check is performed on the whole lightcurve (which very well might contain gaps, if properly segmented), or on the individual segments of length dimseg (which must not contain gaps). This can be tested when applying the function segment_lc_for_psd.
• Most lightcurves are of the form struct{time, rate, error} but foucalc requires struct{time, rate1} (because usually 2 LCs are given to calculate the cross power spectra). However, if one only want to calculate the PSD, foucalc should have a rate field, which default to rate1.