Additional curves functions

[gawela]

We'll require further functions to do all the fancy things with curves, such es extracting pieces of curves and composing several curves into one. Proposed functinos would be (shifted from comments on curves.hpp):

///  extract a piece of the curve as a new curve
///  \param[in] min_time the begin time of the new curve
///  \param[in] max_time the end time of the new curve
///  \param[out] result the sub curve.
 virtual void subcurve(Time min_time, Time max_time, Curve* result) const;

///  join two curves

///  compose two curves

And further propositions.