Fix Histogram filter range for DataSet

In the transition of the Histogram filter to the new filter types, a bug was introduced. The Histogram filter has an option to set the range for which to compute the histogram. (The range of the data will be used instead if none is specified.) This range was properly used when computing the histogram on a PartitionedDataSet. However, when running on a single DataSet, the new filter structure skipped over the partition code that checked to see if the range was specified. (It also skipped over the code that properly computed the range in MPI.)

This change properly checks to see if Execute was called with a single DataSet and updates the ComputedRange appropriately.