Introduce Waypoint classes for describing targets with multiple points.

[yck011522]

This PR introduces a new class similar to Targets that are used for plan_motion(), but now for a sequence of targets. This is used for planning motions with multiple segments similar to the current plan_cartesian_motion().

The intended structure is a "Waypoints" parent class with two childs "FrameWaypoints" and "PointAxisWaypoints". First is used with a list of Frames similar to the current plan_cartesian_motion use case, second is used for 3D printing type of work.

What type of change is this?

• [ ] Bug fix in a backwards-compatible manner.
• [x] New feature in a backwards-compatible manner.
• [ ] Breaking change: bug fix or new feature that involve incompatible API changes.
• [ ] Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [ ] I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• [ ] I ran all tests on my computer and it's all green (i.e. invoke test).
• [ ] I ran lint on my computer and there are no errors (i.e. invoke lint).
• [ ] I added new functions/classes and made them available on a second-level import, e.g. compas_fab.robots.CollisionMesh.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added necessary documentation (if appropriate)

[jf---]

PointAxisWaypoints is exciting!

[yck011522]

@gonzalocasas ping