Return a ditionary mapping time slide IDs to offset
		dictionaries.

def as_dict(self):
		"""
		Return a ditionary mapping time slide IDs to offset
		dictionaries.
		"""
		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