* [misc] Fix missing compositions documentation.
* [gym/common] More generic stacking quantity.
* [gym/common] Add termination condition abstraction.
* [gym/common] Add quantity shift and drift tracking termination conditions.
* [gym/common] Add support of termination composition in pipeline environments.

* [gym/common] Add base roll/pitch termination condition.
* [gym/common] Add base relative height termination condition.
* [gym/common] Add foot collision termination condition.

* [gym/common] More generic actuated joint kinematic quantity.
* [gym/common] Add multi-ary operator quantity.
* [gym/common] Add safety limits termination condition.

* [gym/common] Add power consumption termination condition.
* [gym/common] Add ground impact force termination condition.

* [gym/common] Add base odometry pose drift tracking  termination condition.
* [gym/common] Add  motor positions shift tracking termination condition.

…ces.Dict'. (#819)

…onditions. (#820)

* [gym/common] Add relative foot odometry pose shift tracking termination conditions.
* [gym/common] Add unit test checking that observation wrappers preserve key ordering.

… performance. (#821)

* [core] Fix robot serialization issue.
* [python/simulator] Consistent keyword arguments between 'Simulator.build' and 'Simulator.add_robot'.
* [gym/common] Fix quantity hash collision issue in quantity manager.
* [gym/common] Refactor quantity management to dramatically improve performance.
* [gym/common] Add 'order' option to 'AdditiveReward'.

* [core] Minor improvement periodic Perlin process and periodic stair ground.
* [core] 'PeriodicGaussianProcess' and 'PeriodicFourierProcess' are now differentiable. 
* [core] Fix negative time support for all existing random processes.
* [core] Add N-dimension Perlin processes.
* [core] Add gradient computation for all Perlin processes.
* [core] Make all Perlin processes faster and copy-able.
* [core] Add Perlin ground generators.

---------

Co-authored-by: Alexis Duburcq <alexis.duburcq@gmail.com>

* [core] Replace MurmurHash3 by xxHash32 which is faster.
* [core] Make gradient computation optional for heightmap functions.

…depth). (#826)

* [python/viewer] Add support of user-specified extra cameras (rgb and depth).
* [python/viewer] Significantly speed-up both offscreen and onscreen rendering for Panda3d.