Further work on aggregates

[rnewman]

After #39, the following items remain:

• Inclusion of type tags. Check that aggregation behaves correctly (that is: reject the query!) for values of differing types.
• Type handling for aggregates: e.g., max on instants should yield an instant, not a plain number.
• Constant expressions (Support constant aggregate expressions #585)
• Limits (double-check) ([query] Implement limits correctly when aggregating #316)