Split core library, specific TOPP algorithms into their own packages

[jonbinney]

This will make it easier to maintain the core library. Right now traj has a mix of core functionality and TOPP algorithms. I'd propose the following split for now:

• traj - PiecewiseFunction, plotting tools. maybe also s-parameterization code?
• topp_seven_segment (need a better name) - TOPP algorithms that use seven segment piecewise polynomials as their key building block
• topp_xxxx - future packages for new algorithms we implement.
• traj_testing - depends on both traj and topp_seven_segment and topp_xxxx librarie;, includes the tests that we'll run in CI