feat: adding three new rate_functions based on the SmoothStep function

[uwezi]

The SmoothStep functions have the added benefits of zero derivatives at the endpoints. Since these are simple polynomials with a maximum of 4 terms the necessary calculations are fast and require few resources.
https://en.wikipedia.org/wiki/Smoothstep

Motivation and Explanation: Why and how do your changes improve the library?

The current rate function smooth has non-zero derivatives at the endpoints, because it is a truncate logistics function. This can lead to unexpected jumps in the very first and last frames of an animation.

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[MrDiver]

LGTM!