This modification corrects a bug that appears when using observations only. Indeed, in that case, it is not possible to use the model mask for the observations since there is no model even if the user requested an interpolation. Therefore the condition nmod >= 1 should be tested whatever the user requests for output=lonlat/lat/lon (then an interpolation is needed) or the user requests explicitely for an interpolation. If nmod == 0, the observed mask would be used ( although a security test to check that this mask has indeed been provided could be good to add)