code: def as_dict(self):
  		
  		d = {}
  		for row in self:
  			if row.time_slide_id not in d:
  				d[row.time_slide_id] = offsetvector.offsetvector()
  			if row.instrument in d[row.time_slide_id]:
  				raise KeyError("'%s': duplicate instrument '%s'" % (row.time_slide_id, row.instrument))
  			d[row.time_slide_id][row.instrument] = row.offset
  		return d
docs: Return a ditionary mapping time slide IDs to offset
  		dictionaries.