BUG: Clear Statistics measurement maps when executing

When rerunning the label statistics image filter, old labels would
still be in the measurement map. This clears the map before
execution.

Change-Id: I7c3c5e75b43297b689dcf92789b76c66ab4e2ec0
Merge topic 'SIMPLEITK-417_LabelStatisticsClearMeasurements'

f65e122 BUG: Clear Statistics measurement maps when executing