Sets or returns the right tangent Y of the fcurve key as a
Double. The right tangent Y value
is a positive offset from the current FCurveKey.Value.
The left tangent of the first key and the right tangent of the last
key cannot be changed; if you try to set the tangent for one of
these the property does nothing and returns S_FALSE in C++.
Tangent values only apply to standard function curves (where
FCurve.Type = siStandardFCurve), the
tangent value 0 is returned for all other function curve types
defined by siFCurveType.