Reindent aggregate.rkt from DrRacket
Add simple group/agg-val test for even?
Ignore temporary editor files '*~'
Add tally
Clarify the manual might be out of date during development
Add simple test for aggregate/agg-val - use all aggregates for (range 10)
Add test for group/agg-val

Putting everything into the same group should make the final aggregate
values be the same for both aggregate/agg-val and group/agg-val.

The test fails because group has a bug - it doesn't finalise the
aggregates.
Make group finish aggregates - fixes bug and makes group/agg-val test pass