Added aggregate and coordinates methods

I've decided for a coercion to SpatialPointsDataFrame, since building upon STIDFs didn't work properly. I'll check that later. For now, aggregation over space and time works fine for all Track, Tracks and TracksCollection objects:

```
library(spacetime)
example(Track)
crs = CRS(proj4string(Tr))
# I'd suggest to let bbox() return the bb in the same structure as it does for
# sp objects, unless there's an obvious reason for the transposition of rows and
# columns as is currently implemented?!
bb = bbox(Tr)
grd = GridTopology(as.numeric(bb[1, 1:2]), apply(bb[1:2], 2, diff)/5, c(5, 5))
sp.agg = aggregate(Tr, SpatialGrid(grd, crs), max)
spplot(sp.agg[1], colorkey = TRUE, sp.layout = list("sp.points", Tr, col = 3),
main = "Maximum measured value per grid cell")
```

A coercion to SpatialPointsDataFrame now preserves any temporal information. I've also added coordinates methods, which are required to make the aggregation methods work properly.

The code can be easily extended to allow for aggregations over the connections slot (still have to decide how to shift, though).