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.