Behaviour commands: body height on terrain

A stronger height reward makes the body-height command work, and a body held lower crosses less terrain. Most of those failures were a front leg's sideways joint at its mechanical stop. The robot clamps every joint command 0.1 rad inside its stop, which the simulator had not done; with that clamp our walks are unchanged.

• 13 cm body-height command: 15.9 / 15.1 / 13.9 cm as the height reward grows; passes on 3 cm stairs, 15° slope and 5 cm curb fall with it, 76 / 58 / 37 of 90 (one evaluation)
• Failure before the goal: mostly a front leg's sideways (abduction) joint at its 0.42 rad stop, not the knee
• Robot's own limits (0.1 rad inside each stop) in the simulator: stair policy 86 → 90, multi-terrain walk 88 → 88 of 90
• Trained with those limits, the 14.7 cm crouch rests a front leg on its stop where the earlier policy tipped over: 71 of 90 if that counts as no fall, 49 if it does
• Decided Oct 19: a front leg may rest on its sideways stop while walking, as on the robot, so evaluations count it as no fall