Should a Union type be here or just one type? https://github.com/harmoniqs/DirectTrajOpt.jl/blob/8ea59c46889f5dbd8fa1a0668c165e14c00e1cc7/src/objectives/_objectives.jl#L34 Do we want to avoid abstract vectors? https://github.com/harmoniqs/DirectTrajOpt.jl/blob/8ea59c46889f5dbd8fa1a0668c165e14c00e1cc7/src/constraints/nonlinear_constraints.jl#L10