add smooth_polynomial function

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Request from Trond:

I’d suggest adding one more easing function from Jamoma, even if it’s not one of Penner’s easing functions: smoothPolynomial. This function has growth rate = 0 and curvature = 0 at the end points, and hence has a completely smooth way of starting and stopping. It might look pretty similar to e.g. the cubic ease in / out, but when using it to ride audio values in order for layers to come and go as drifting waves, there is a difference, and smoothPolynomial sounds really nice.