Exponential approach tool

Rough tool to map equations of the form value_n = value_n-1 + (target - value_n-1) * expFactor
which depend on the number of iterations, to ones that consume time deltas and behave a bit better with varying framerates value_n = target - (value_n-1 - target) / e^{dt / timeHorizon}

[here's a more detailed write-up on this]

Given expFactor and a framerate, you can adjust timeHorizon to make the functions match (for example, timeHorizon = 0.1494 matches expFactor = 0.2 with 30 fps). Support for different implementations of e^x is provided, though in practice a 3rd order Taylor series works well enough. The analytical solution only guarantees accurate results when using Math.exp.