This PR seeks to bring habitat-API inline with this. StaticPointGoalSensor is now just a PointGoalSensor and the old PointGoalSensor is now a PointGoalWithGPSCompassSensor. There are also now explicit GPS and Compass sensors, such that you can fairly easily compute PointGoalWithGPSCompassSensor given [PointGoalSensor, GPSSensor, CompassSensor].
The PointGoal* sensors now also include {2d, 3d} x {Cartesian, Polar} support.

* before doc

* use iter getter instead of as a dataset attr

* fix docstrings

* changes according to comment

* fix long var name

* use default value in next

* Add fog-of-war to topdown map

* Add checkpoint progress tracking for evalute_ppo. Now when specified with a checkpoint directory,
* Evaluate_ppo will evaluate checkpoints in chronological order, and constantly check for new checkpoint.
* Add tensorboard visualization to both train_ppo and evaluate_ppo
* Add video generation for navigation episode evaluation. Generated videos can be either saved locally or visualized through tensorboard.
* Add shortest path visualization

SimulatorActions is no longer an enum. It is an extendable singleton. It allows you to extend the action space but it is quite strict about it. Actions cannot be put on different ints, actions cannot be remove.
There is now a registry for action space configurations.

* Generalize PPO with both PointNav goal sensors
* generalize simple agents and fix lint
* Removed dataset specific nodes

Adds documentation for habitat-api, to view the docs follow instructions in docs/README.md and view the html files from the build folder.

- add config option NUM_SAMPLE_EPISODE, with default value -1 that
indicates use all episodes. When set value to non-negative integer,
pointna_dataset will sample the specified number of episodes.
- fix the functionality of merging multiple config files. Now can specify
more than one config file like this
	--task-config "a.yaml" "b.yaml"

Changing action space names to say what they actually do, adding the look up and look down actions.

  * Add `test_static_pointgoal_senosr()` in `test/test_sensor.py`
  * Fix overriden configs in `test_sensors.py` (now using default config)
  * Change default config to include static pointgoal sensor
  * Fix `StaticPointGoalSensor._get_sensor_type()` to return `PATH`

Config system improvements: compositional config, added scenes_dir, removed config_dir, fixed eqa test (#77)

* Added scenes_dir parameter to Config to have no requirement of data symlink or data directory in working directory. scenes_dir will be used as prefix for scene_id during episodes dataset loading.
* Added overwriting options argument to get_config that unblocks parameter sweeping and experimentation capabilities from command line
* config_file path can be comma separated list of configs that will be merged in exact order. That will help to use separate configs for dataset specification and sensors setups. String with commas was chosen because that make it easy to pass list of configs as script argument.
* To make habitat-api easier to as package we decided to get rid of forcing loading of configs from config_dir param
* fixed unit test caused by latest habitat-sim update
* Added support of config list, corrected scene_path, added overwriting options, added EQA dataset changes, adopted quaternion suggestion
* Fixed headers

* static PointGoal sensor, Top Down map measure
* top down map measure

Added collision and proximity measurements; Use is_navigable for a more robust check for top-down map

Benchmark and challenge implementations; PPO agents, Random, ForwardOnly, GoalFollower agents; High resolution top down map and bird-eye visualizations; Collisions measure introduced; SPL success criteria fix; Added deleter that closes subprocesses