Period search margins and algorithm

[svaverbe]

The lower margin for the period search should not be set much above the Nyquist frequency of the dataset. For the IC5070 data, taking 0.5 d as the lower limit and t_base/2 as the upper limit avoids many wrong periodicities. By switching to the Generalized Lomb Scargle Periodogram, a lower limit for 0.5 can taken as a conservative lower limit for a strong periodicity with a negligible FAP. Lower peaks should be carefully checked by looking at the lightcurves. I have uploaded a python code which uses the above suggestions to compute the periodograms for thz IC 5070 data.