Polynomial trajectory class for the warm starting MPC with the variable problem structure

[mayataka]

In MPC for the legged robots, the problem structure of the OCP can change due to its discrete nature. A little a bit complicated warm start strategy may be useful. A class that represents the polynomially approximated continuous-time trajectory is useful for such cases, e.g., as

1. Solve an MPC problem
2. Construct the polynomially approximated continuous-time trajectory from the MPC solution
3. Proceed to the next sampling time, and then the problem structure of the OCP can change.
4. Guess the initial solution of the new MPC problem by the polynomially approximated continuous-time trajectory.