Targets Classes for plan_motion and plan_cartesian_motion

[yck011522]

Creating Targets Classed for the lowest level planning API to consume.

Some follow up PR will modify the plan_motion and plan_cartesian_motion API to take this new class type. The goal of this is to have a slightly higher level of abstract (Frames / Config) instead of the current practice of converting the targets to list of Constraints for the planning call.

This will allow the PyBullet planner, which does not support Constraints to understand the Frame and Config targets.

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)

[yck011522]

I made a mess. Will do the branch again.