aggregation ST by Spatial now aggregates over time

aggregation used to be by time slice too, making it only
useful for STFDF objects, and actually doing an aggregate(ST,ST)
where the second has identical time resolution as the first.

The old behaviour is retained by
aggregate(STFDF, Spatial, keepTime = TRUE)
which is NOT the default, now (so I need to adapt sto.Rnw too).
Merge branch 'master' of github.com:edzer/spacetime