[wip/py-croccodyl] Update to v0.2.0

Changes since v0.1.0:
- Python 3 & CI
- Add the frame rotation cost + improve description of cost classes
- [Format] Update yapf to 0.27.0
- Removed wrong assert in `CostModelControl`
- Save computation time in DDP and FDDP solvers + fixed bug in DDP
- Fixed bug in `quasiStatic` function of `DAMFIC`
- Reformat, and enforce format
- Do not try step if there is invalid direction (DDP and FDDP)
- Updated the REAME file + cleaned up references
- locomotion module refactoring.
- Created solver abstraction and implemented for the SolverDDP, SolverFDDP and SolverKKT
- Standarized unit-test for NumDiff
- boxddp
- Ensuring symmetric in Vxx, this is needed because numerial error
- Fixed an error in the equations for feasible_ddp document, and improved it too