Add smoothstep as an alternative to exponential approach

[Not-A-Normal-Robot]

https://en.wikipedia.org/wiki/Smoothstep

See also: https://easings.net/

[MrZ626]

Exponential approaching has self similarity, expAppr(expAppr(x,1),2)==expAppr(x,3), this is a important property, I can make animations without saving any states
But smoothstep approaching need to save states, it's impossible to make a single-func implementaion
A new big module TWEEN including many easing curves will be created if you really want it