Add min_jerk_trajectory() and improve self-test

[luator]

Description

• Add Python generator min_jerk_trajectory() (in utils module) to easily compute smooth trajectories. Use it in trifingerpro demo and in self test.
• trifingerpro_post_submission: Use a non-contact reference value for testing the push sensors instead of just comparing to a hard-coded threshold. This should reduce the likelihood of false positives.
• trifingerpro_post_submission: Speed up a bit and smoothly move to start of reset trajectory to avoid jerky movements.

How I Tested

on a robot.