Add audio testing examples in sandbox. Add resources to the data folder

Co-Authored-by: Alexis Girault <alexis.girault@kitware.com>

* Add data needed for the examples inside Sandbox
    except ETI & textures
* Remove condition on DATA_ROOT_PATH since the user doesn't have to specify it
* Rename DATA_ROOT_PATH to iMSTK_DATA_ROOT according to 267b7f72

Download Data from Girder according to data key files contained in iMSTK/Data

This uses ExternalData.cmake (Requires CMake >= 2.8.11 release).

Add TEST: Add ExternalDataTest to verify downloaded files from Girder
* Be sure the data requiered for the test is not there (delete it)
* Add the imstk_add_data(${target} ${data_list}) function in CMakeList.txt
which calls ExternalData_expand_arguments().
* It downloads the data provided by the data_list (relative path from
iMSTK/Data)

Add DOC: Add documentation on ExternalData Management
* How to add data to iMSTK from Grider
* Update data
* Use data