Strip both leading and trailing underscores from MuJoCo API identifiers.

PiperOrigin-RevId: 412862961
Change-Id: Ifa490065fac8b7316a19856880dcea050803e0e0

Fix upright position for CMUHumanoidPositionControlledV2020.

Closes google-deepmind#192

PiperOrigin-RevId: 387317115
Change-Id: I9025c8329597558a5f15186956862628721d7daf

Internal change

PiperOrigin-RevId: 321746460
Change-Id: Ib96ce14bd217d7b7e7b3bf28442e22e85699a2d9

Internal change.

PiperOrigin-RevId: 320948823
Change-Id: I3b48b1c4d5b7c6a4ff7c4864ca4d62e39d698704

Final commit before upgrading to MuJoCo v2.00