obtain ranges of space and time coordinates
stbox(obj)object of a class deriving from Tracks or TracksCollection.
stbox returns a data.frame, with three columns
representing x-, y- and time-coordinates, and two rows containing min
and max values. bbox gives a matrix with coordinate min/max values,
compatible to bbox
signature(x = "Tracks"): obtain st range from object
signature(x = "TracksCollection"): obtain st range from object