tmf: Move statevalue unboxing method implementations to the base classes

tmf: Move statevalue unboxing method implementations to the base classes

Made the default methods in TmfStateValue always throw exceptions.
Implementations can then override those for the types they support. Also
added some messages to the unboxing exceptions, to help when debugging.

Tweaked IntegerStateValue's so that they can be unboxed into a long,
since this will always be safe. The opposite however is not permitted.

Change-Id: I492a53e8daf1141c8aefa1c0ad1f005af8e77c30
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/16294
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>