ENH: expose bounding box label statistic

A separate member/measurement variable is added to the
LabelStatisticsImageFiter, which store the Bounding box as an array
just the ITK, although as a int not a unsigned long or what ever ITK
is configured for.

Change-Id: I78270536f9ce2472f3c4f8daf6a69e9bfecfd488
ENH: adding test for bounding box in label statistics

Change-Id: I27a1ee3546a762f28c84dd27487cb7d30b8d6343
Merge topic 'AddBoundingBoxToLabelStatistics' into next

f2fe063 ENH: adding test for bounding box in label statistics
4ce7ab2 ENH: expose bounding box label statistic