Merge pull request #5 from k2sk8s/master

Added aggregate and coordinates methods

Note that when coercing to SpatialPointsDataFrame, aggregate methods will not know how to deal with time (temporal aggregation, or spatio-temporal aggregation).
update date and role
coerce SpatialGrid into SpatialPixels in aggregate
add contribute file, and .Rbuildignore
Update contribute.md
Update contribute.md
Update contribute.md
Update contribute.md
Update contribute.md
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
add stbox method, set back bbox, correct aggregate

stbox methods now give the space and time ranges as a
data.frame; bbox gives the sp-compatible spatial range matrix.

aggregate with only a Spatial argument can do aggregation
over all time, or spatial aggregation by time. STF and STF
do it default per time slice (as in the sto vignette), other
classes will group all, within a spatial feature, over time.
Added generalize method.