[C++] refactor hpp-spline -> curves

include/hpp/bezier-com-traj/common_solve_methods.hh

include/hpp/bezier-com-traj/data.hh

include/hpp/bezier-com-traj/definitions.hh

include/hpp/bezier-com-traj/utils.hh

src/common_solve_methods.cpp

src/computeCOMTraj.cpp

src/solve_0_step.cpp

src/utils.cpp

tests/test-bezier-symbolic.cpp