Bug#355173: gweather: doesn't understand NCD metar cloud group

[Ross Younger]

Package: gnome-applets
Version: 2.8.2-3

The NCD cloud group (No Cloud Detected by automatic observation) is
reported as "Invalid" by the applet. Here's an example METAR which
tripped it up: 
        EGUB 031750Z AUTO 02002KT 9999NDV // NCD M01/M05 Q0998

I suggest it be treated the same as NSC, i.e. set internally as
SKY_CLEAR, on the basis that that's the best you can do given the
information available.