change key of NewRowToOldRowsMap to a vtkVariant

This fixes a bug where the wrong rows in the input table could be
used to calculate a value in the output table.

Change-Id: Ibd5dac710f3a8ea1e19971c8fbd13a5742aa86a6
eliminate unused variable warning

Change-Id: I45a40f94ee78f3a696bfebdd66374aa4d4be0dbd
Merge topic 'fix_reduce_table_bug' into master

e86c7af6 eliminate unused variable warning
772916a4 change key of NewRowToOldRowsMap to a vtkVariant