ENH: Adding UseHistograms options to LabelStatistics

Change-Id: I2e3217f55a856288d04644cce058bac188dfe10a
Add initialization to the new parameter member variable.

Change-Id: I2efcdd002e9ed49371452518c95ea37a2348c3a1
Adding UseHistograms option to procedure and Execute methods

Change-Id: I90b0438b6efed1932c69614ed3f8ad0e1c15a153
COMP: fix extraneous semi-colons at end of member function definitions

Change-Id: I3aa7585be419cd2f9cf883ae85da00147968b331
Merge branch 'SIMPLEITK-510_AddUseHistogramsOptionToLabelStatistics' into release

Change-Id: I0e5d3ea297853e6ceb924f82dab7db69dc8083d5