[API] boundary values for x variables in optimization

include/bezier-com-traj/common_solve_methods.hh

include/bezier-com-traj/solve.hh

include/solver/glpk-wrapper.hpp

include/solver/solver-abstract.hpp

python/bezier_com_traj.cpp

src/common_solve_methods.cpp

src/computeCOMTraj.cpp

src/glpk-wrapper.cpp

src/solve_0_step.cpp

src/solver-abstract.cpp

tests/test-transition.cc