Allow access to bounds on local descent

[hjsuh94]

For position controlled systems, it could be the case that ubound depends on a nominal point on x_trj, but the current code structure only allows uniform bounds.

[hjsuh94]

One solution might be to accept a u_bound_relative parameter that becomes additive on state.

[pangtao22]

I'll give it a stab when I PR the contact stuff.

[hjsuh94]

After trying out many examples, the following four bounds will cover everything:

• x_bound_abs: this helps when there are actual bounds in the system (joint limits, ground, etc.)
• x_bound_rel: this helps to establish a trust region around current nominal trajectory.
• u_bound_abs: this helps when the system is force / velocity controlled.
• u_bound_rel: this helps when the system is position controlled.

In addition, quasistatic systems put a cost on du instead of the absolute u, which might also be problematic.