Add PointAt constraint

[bruingineer]

#381
along with SleipnirGroup/TrajoptLib#140

[calcmogul]

I've confirmed this works. There's a lot of gotchas around when the constraint is valid to use tho, vs immediately making the trajectory infeasible. In my test, I had to make a translation waypoint between two pose waypoints, then add the point-at constraint to the translation waypoint only.

It would be nice if there was a separate point the user could drag around that represented where to point at, but we can do that later.

[calcmogul]

A requirement when using this seems to be to bracket the constraint with two translation waypoints, so the heading has time to transition between a point-to region and a pose waypoint after it. Using a sufficiently large heading tolerance is also important. Seems to work fine though, once you have those two things.