Restore code to set histogram to exact for 8-bit data types

Add the custom code to set the histogram range to be exact for uint8
and int8 data types.

Change-Id: I17bebf7ee173c643aed4d246d8f4d6211f4a1264
Release private filter reference before PreUpdate

The old filter should be released before the PreUpdate method is
called. This causes the Delete event to occur and clean up to
happen. This fixes re-execution of filters with active measurements.

Change-Id: I51a085e48bb8551223f22e730762fe92a5e13305
Merge topic 'SIMPLEITK-490_Uset1functionForLabelStatisticsImageFilter' into next

ae972289 Release private filter reference before PreUpdate
5d7985b3 Restore code to set histogram to exact for 8-bit data types