Support trimming an existing trajectory

[lidor51]

This PR supports a trimming of a trajectory. Introduce an elegant way to start a trajectory in the middle (and also to terminate it earlier).

My original thought was to trim a PathPlannerPath, but it would be less user friendly as the params were WaypointPos instead of seconds.
Let me know if you think it's better to add trim support to a path instead to a trajectory.

#764
#698

[mjansen4857]

This isn't something that I think should be in the main library. Trimming trajectories like this can lead to some pretty bad path following performance very easily. If you cut out part of the trajectory in the beginning handling acceleration, the robot would be commanded to go much faster than it is capable of doing and the robot will fall behind for the whole trajectory.

This method is something that can be easily implemented in user code, and if you know what you're doing when using it then it can work fine, but if its not something I would recommend anyone to use because of the potential issues caused by it then it just probably shouldn't be in the library. I want to leave anything like this outside of just following the trajectory as-is up to the user to implement themselves so they are fully aware of what is happening.

[lidor51]

If you can expose also getters for PathPlannerPath, it'll be appreciated, so advanced users can make modifications to the path without modifying the json object in runtime. Do you want me to open a feature request? @mjansen4857

[mjansen4857]

What getters? I think everything needed should already be exposed.

[lidor51]

Getter of number of waypoints (not path points) & getter of rotation targets (although can be extracted from iterating through path points), would be appreciated.

[mjansen4857]

Yeah you should be able to trim the path purely off path points but all that stuff should be publicly accessible anyway so I’ll do that.