improve star detection

This MR aims at improving the detection when the star kernels are large.
This is done by adding some more initialization steps in PSF.c, when the initial assessment shows that FWHM is above a given value.
When this is the case, an additional step is performed to provide as well estimates of the roundness and angle (instead of using FWHM about x and y axes which are not relevant). The computation entails computing the moments (up to the second) of the area above the half amplitude and use these numbers to compute FWHM along the major axis, roundness and angle of the major axis wrt. x.

Still to be done some checking that:

• performance is not impacted (shold not as the extra steps made during init should help the optimization converge faster, or just converge)
• overall detection is not impacted on systems without oversampled stars
• also need to check the photometry_test (testing fails for now)