Qibullet

* Solving 84
* Solving 85

Changelog:
* Solving compatibility with recent pybullet version, causing the ROS wrapper to fail. (fixing 79)

Changelog:
* Better handling of error raising for the FSR
* Improving code coverage, and update coverage launch commands in CI
* Specifying Python 3.9 compatibility, solving 64
* Updating the example snippet in the README for more clarity, fixing 69
* Migrating from Gitter to Github discussions (update badge in the README), solving 73
* Updating Doxygen doc (optimized for Python, and new doxygen theme added), solving 72
* Auto generating and deploying doxygen doc from the CI when creating a new qiBullet version, also in the scope of 72
* Automatically packaging and deploying qiBullet to PyPI test and then PyPI when creating a new version

Changelog:
* Add manual stepping for the simulation, both in GUI and DIRECT mode, solving 50
* Migrate CI from Travis to Github actions
* Fix `getLinkPosition`, solving 57
* Adding an IMU to Pepper, NAO and Romeo, partly solving 61
* General refactoring & optimization, removing spurious code
* Add a way to customize the framerate of a camera when subscribing to it
* Python 3.9 compatibility, solving 60
* Adding a way to set/get the gravity for a specific simulated instance
* Adding FSRs for NAO, solving 61
* Add / update existing examples, and update the qiBullet wiki, solving 61 62
* Optimize the installers

Changelog:
* Refactor the ROS wrapper classes
* Handle multiple active cameras in the ROS wrapper classes
* Update the Pepper laser example, solving 39
* Fix the robot spawning methods docstrings, solving 43
* Adding a `stopMove` method, solving 40
* Adding the `getLinkPosition` method, solving 41
* Adding the `getJoint` and `getLink` methods, allowing the user to retrieve links or joints without explicitly using the `joint_dict` and `link_dict` directories
* Update the unit tests

Changelog:
* Refactor the camera subscription system, using handles (more than one camera can be used simultaneously)
* Refactoring `getAnglesPosition`, solving 28
* Refactoring `getAnglesVelocity` solving 29
* Clarifying ROS driver import message, solving 30
* Update unit tests
* Improve coverage

> :warning: The camera subscription system of qiBullet 1.4.0 (and lesser) is __deprecated__, use the [new system](https://github.com/softbankrobotics-research/qibullet/wiki/Tutorials:-Virtual-Robot#cameras)