Create a "Visualizers" library and add the existing "robot visualizer" block

[nunoguedelha]

In order to use the simulator in other frameworks, typically within the controller models in whole-body-controllers (refer to whole-body-controllers#121, we need to convert the robot visualizer into a library.

[nunoguedelha]

We create a main library mwbs_lib.slx to which we link other sub-libraries robotDynamicsWithContacts_lib.slx and visualizers_lib.slx (newly created), which wil be visible in the library broaser as follows:

RobotDynWithContacts	Robot Visualizer

The RobotVisualizer block is the new one we create here from the pre-existing block test_matlab_system_2020b/Robot visualizer/robotVisualizer

For creating a tree of sub-libraries in a library, refer to https://www.mathworks.com/help/simulink/ug/adding-libraries-to-the-library-browser.html.

[nunoguedelha]

Implemented by #41 .

[nunoguedelha]

CC @traversaro