December 2020 release

New Levels:

1. Psychlab.

   1. contributed/psychlab/memory_suite_01/explore_goal_locations_extrapolate
   2. contributed/psychlab/memory_suite_01/explore_goal_locations_holdout_extrapolate
   3. contributed/psychlab/memory_suite_01/explore_goal_locations_holdout_interpolate
   4. contributed/psychlab/memory_suite_01/explore_goal_locations_holdout_large
   5. contributed/psychlab/memory_suite_01/explore_goal_locations_holdout_small
   6. contributed/psychlab/memory_suite_01/explore_goal_locations_interpolate
   7. contributed/psychlab/memory_suite_01/explore_goal_locations_train_large
   8. contributed/psychlab/memory_suite_01/explore_goal_locations_train_small

2. Language binding tasks.

   1. contributed/fast_mapping/fast_mapping
   2. contributed/fast_mapping/slow_mapping

New Features:

1. A property system has been added that allows dynamic querying and modifying of environment state. Level scripts can register and consume custom properties.
2. A new Python module, dmenv_module, is provided that exposes the DeepMind dm_env API.

Minor Improvements:

1. Quake console commands can now be issued via a write-only property.
2. New numeric "accumulate" operations for TensorView and the Lua Tensor types: sum, product, sum-of-squares, and dot product of two tensors.

EnvCApi Changes:

1. "Properties" have been added to the EnvCApi. Properties may be queried, set, and enumerated.
2. The new API version is 1.4 (up from 1.3).
3. The EnvCApi function fps is now deprecated; environments should instead use the new property system to communicate this information.

Bug Fixes:

1. Fix observation 'VEL.ROT' to allow non-zero values when combined with pixel observations. Previously, the presence of pixel observations caused the angular velocity information to be lost due to a logic error.