fourth-order moments

[martinkilbinger]

Summary

Implementing fourth-order moments.

Solves #697

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[sachaguer]

@martinkilbinger I modified the psfex script to compute the 4th order moments. I checked it runs on the star vignets but not on the interpolated PSF. The easiest is probably to rerun this part of the code like you did already and check if it saves correctly the 4th order moment information.

I will copy and paste the script in the corresponding functions for the MCCD equivalent.