Arm interface

[ennerf]

This PR provides a higher-level abstraction for working with (serial chain) arms. The main features are:

• moving along trajectories with gravity compensation and pos/vel/effort control
• seamless replanning of trajectories
• keeping track of auxiliary (e.g. gripper) state
• a plugin system that allows adding e.g. impedance control as needed

This PR also includes changes to the API itself, which will be released separately if this PR gets approved

• minMoveTime now applies in between multiple waypoints and not only on the entire duration (this behavior was intended for 2 waypoints, so this can be considered a bugfix)
• HebiUtils.sendWithRetry

[ennerf]

I updated it to the official 1.7.1 matlab release, so I'm ok merging it

[ennerf]

Thanks for reviewing it. I created a new namespace, so plugins show up in autocomplete now, e.g.,

plugin = HebiArmPlugins.EffortOffset(offsets)