Update an attribute in ncout

Update an attribute in ncout

update_attribute(ncout, var, key, val, prec)

Arguments

ncout	file handle for netCDF in which attributes should be updated
var	the name of the variable to which the attribute is associated (or NULL for a global attribute)
key	the name of the attribute
val	the value of the attribute
prec	the precision of the attribute (or NULL to use the same precision as var)