The filter classes have an internal CellSetIndex member that tracks on
which cell set to operate on. The get accessor is called
GetActiveCellSetIndex (note the descriptive "Index" at the end of the
function name). However, the set accessor was called SetActiveCellSet
(sans "Index"). This discrepancy does not make a lot of sense.

This commit changes SetActiveCellSet to SetActiveCellSetIndex. Not only
do I like the extra descriptor (in case we later want to set cells by
name), it is also used much less than the get method so is less
disruptive.