The function coarsenGrid is a function that samples from a
SpatialGridDataFrame.
The argument coarse indicates that every coarse row and column will
be sampled, starting with the row and column represented in offset. offset = c(0,0)
implies that the smallest x- and y-coordinates will be a part of the resampled
data set, offset = c(1,1) implies that sampling will start on the second row and column.