This function will scan through changeList, and if an item in
changeList exists in originalList, but both items data values
are equal, then the item in changeList will be removed from the
list.
If the value of apply is TRUE, then if the datas are different
then the values in originalList will be updated to match those
in changeList.
INPUTS
changeList
List of new tags (may be NULL).
originalList
List of existing tags (may be NULL).
apply
Boolean flag as to whether the values in
originalList should be updated to match
those in changeList.
RESULT
The changeList will be modified to show altered items, and if
requested, the originalList will be updated.