[Bf-python] IPO curves evaluation

[Carlos López]

Yeah, i know, there's the new method 'getCurvecurval',but I have not a good control over the curves.
I need to evaluate a curve at any point, not only entire numbers (ie, frames): I must use them to calculate derivatives to measure velocities, etc., so I make:

y1=evaluateCurveOn(t-0.001)
y2=evaluateCurveOn(t+0.001)
velocity = (y2-y1)/0.002
..

Now I'm not able to do that, anyway!