Either a single value of the total area if byCell=FALSE, or a matrix
the same shape as m of individual cell surface areas if
byCell=TRUE. In this case, the sum of the returned matrix should be
the same value as that which is returned if byCell=FALSE.

Missing values (NA) in the input matrix are allowed. They will produce
an NA in the output matrix for byCell=TRUE, and contribute zero to the
total area. They also have an effect on adjacent cells - see code
comments for details.