Sets or returns the right tangent X (in frames) of the fcurve key
relative to the FCurveKey.Time value as a
Double.
The right tan x value may be a positive value between a distance
which is one third the distance between 2 consecutive key and 0;
this guarantees that the curve is valid.
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.