Uniform CMake dependency handling to the YARP/ICUB style

[traversaro]

Currently, all dependencies in iDynTree are searched or not based on the IDYNTREE_USES_<dep> CMake option. This is convenient for its simplicity, but quite cumbersome because a user needs to select explicitly the dependency that he wants to enable. This do not apply to superbuild, but it applies to people building the library standalone.

I am against always enabling the dependency automatically if they are found, because it prevents the user to manually select the dependency that he wants to build, and it prevents to have a clear and nice error if a dependency requested by the user is not satisfied.

I think a good trade-off is the one used in YARP/ICUB, in which the initial value for the IDYNTREE_USES_<dep> option is set to TRUE if <dep> is found on the system, and FALSE otherwise. Then the user is free to select/deselect the option that he/she prefers.

[traversaro]

Done in #323 .