TMF: Add get parent to state system

TMF: Add get parent to state system

It is possible to get the children of an attribute. This patch add also
required methods to query the parent attributes.

* Add getParent() method to Attribute, AttributeTree and StateSystem
* Add unit test to check the functionality
* Provides the required documentation

Change-Id: I635326068c2a298b32952599e09b2426b2e1fbb0
Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>
Reviewed-on: https://git.eclipse.org/r/24026
Tested-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
IP-Clean: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
IP-Clean: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>