This type represents a CURVE.


A curve is a type of geometry (PK_CLASS_curve is a subclass of PK_CLASS_geom).

A curve is of one of the types:

 - PK_BCURVE_t:  a B geometry curve
 - PK_LINE_t:    an unbounded straight line
 - PK_CIRCLE_t:  a complete circle
 - PK_ELLIPSE_t: a complete ellipse
 - PK_SPCURVE_t: a Surface Parameter curve
 - PK_FCURVE:    a Foreign curve
 - PK_ICURVE_t:  an intersection curve