returns a dataset with zero where the data is invalid, and positive
non-zero where the data is valid. (This just returns the weights
plane of the dataset.)
r= where( valid( ds ) )
Parameters
ds - a rank N dataset that might have FILL_VALUE, VALID_MIN or VALID_MAX
set.
Returns:
a rank N dataset with the same geometry, with zeros where the data
is invalid and >0 where the data is valid.