Preliminary implementation of a RESCALE_WEIGHTS configuration switch.

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.

Added  POLAR1ERRMODEL_WORLD,POLAR2ERRMODEL_WORLD,POLARCORRMODEL_WORLD,ELLIP1ERRMODEL_WORLD,ELLIP2ERRMODEL_WORLD and ELLIPCORRMODEL_WORLD measurement parameters.

Removed singular matrix warning.
Added MATRIX_MODELERR measurement parameters.
Added DURATION_ANALYSIS measurement parameter.