Add lib for n-dimensional cubic curve fitting

This will be used for calculating bezier curves from freehand drawing,
may be used for other areas too.

Original code from GraphicsGems, 1990 (FitCurve.c),
with updates from OpenToonz, under 3 clause BSD license.
with own minor modifications for integration with Blender:
- support adding extra custom-data.
- improved handle clamping.

doc/doxygen/doxygen.extern.h

@@ -11,6 +11,10 @@
  *
  */
 
+/** \defgroup curve_fit Curve Fitting Library
+ *  \ingroup extern
+ */
+
 /** \defgroup bullet Bullet Physics Library
  *  \ingroup extern
  *  \see \ref bulletdoc