tmf: Add the Histogram query API to ITmfStatistics

tmf: Add the Histogram query API to ITmfStatistics

See the added Javadoc for the complete description. This will
allow the Histogram view to easily plug into the trace's
ITmfStatistics object to get its information (so it can be
optimized for each trace type, etc.)

Change-Id: Ib94aeaa369a8bf40da5246825152e26326a45e92
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/8675
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>