Preliminary implementation of a RESCALE_WEIGHTS configuration switch.

Fixed issue with output Check-image headers when reading individual extensions from MEFs (thanks to user Lantern for the bug report).

Added more complete headers to all files.
Clarified licensing issues.
Updated FITS library.
Updated configuration files.
Added configuration files to the list of data that get installed in ${prefix}/share/sextractor.
Added background offset and point-source model components: FLUX_BACKOFFSET,FLUXERR_BACKOFFSET,FLUX_POINTSOURCE,FLUXERR_POINTSOURCE,MAG_POINTSOURCE,MAGERR_POINTSOURCE.
Tweaked model-fitting convergence parameters.
Fixed catalogue source ordering/number issue in dual-image mode.
Pushed version number to 2.13.0.

Reverted minimization algorithm back to double precision (while keeping single precision for data and parameters in "bounded" space).
Added specific handling of "Jacobian calls" in model-fitting.
Sped up the resampling process in model-fitting.
Tuned up minimization algorithm in model-fitting.
Added shortcuts to the computation of the local PSF model. 
Fixed triggering issues with some model-fitting parameters (thanks to S.Serrano).
Removed CLASS_STAR_MODEL and DISK_PATTERN* measurement parameters.
Added automatic tracking of the PSF FWHM from the local PSF model if SEEING_FWHM is 0 for CLASS_STAR.
Fixed excessive WIN parameter computation time on large ring-shaped objects.
Fixed model-fitting issue with null half-light radii (thanks to S.Desai and B.Armstrong).
Fixed aperture display value error in FITS-LDAC catalogue header.
Removed redundant update_tab() calls in FITS library.
Fixed comment copy issue for slashes within strings in FITS library (thanks to F.Schuller).
Added check of PSF values; SExtractor will now exit in error in the case where all PSF components sum up to 0.
Replaced computation of model second moments and ellipticities with analytical estimates.
Improved the accuracy of peak and average model surface brightness estimates.
Fixed some compiler warnings.
Pushed version number to 2.11.7.