Sequential tagging of turbOnset, VOT, turbOffset & vowelEnd

This update allows the tagger to sequentially tag the turbOnset, VOT, turbOffset & vowelEnd, in that order.  When tagging the turbOnset, VOT & vowelEnd, markers are automatically placed in the TextGrid, and the tagger is prompted to move them.  When tagging turbOffset, instead of automatically inserting a marker, the tagger is prompted to move the cursor to where the turbOffset would go, if there is aspiration.  The script then determines whether inserting a turbOffset marker is necessary by evaluating:

turbOffset < ((turbOnset + VOT) / 2) + 0.02

If turbOffset is less than the righthand side of the above inequality, then a turbOffset marker is inserted at the cursor's position.