[ctioga2] factor some bit of code in the curve generators
[ctioga2] first working implementation of histograms, very much like the old ctioga

There is still quite a bit of work :-)
[ctioga2] basic decent implementation of histograms

Many style features are still missing, but it does not look that bad !
[ctioga2] tests for histograms and fill styles
[ctioga2] now using fill styles for histogram boundaries too
[ctioga2] a xcolor-style mixing for color sets too !