Remove c11 option for C code (getline warning) and replace M_PI by PI definition (not defined in C11)

Update example_with_arcletstat and example_with_arcletstat_sm. Fix bug in sm_server.cpp recv() flag.

Implement sigma_ap correction (Bergamini+2019). Define DEBUG symbol in --enable-debug mode. Add ./tests directory.

Add a file to generate a B-spline perturbation from a multiple images catalogues as defined in Beauchesne et al. 2021

Correction of the bug relative to global variable defined more than one time. I added the -fcommon options in src/Makefile.am and utils/Makefile.am and I runned automake for Makefile.in files. The codes compile well with gcc11.

Remove useless keywords that are printed in the best.par and bestopt.par files for the Bspline potential

I correct the issue with the Bspline example that were not present and I added a pdf file that I made initially for the article as it was destinated to end-users

This reverts commit 7c6525d8.