G-AIRMET processing tweaks

[akrherz]

Received some expert guidance:

• In an icing GAIRMET, there are usually a top altitude and a base altitude to define the extent of the icing threat. Sometimes the base altitude is just the "freezing level". Also, icing always has a severity (which is always moderate).
• Similarly, there is usually a descriptor for LLWS threat, something like "AIR SPEED LOSS OR GAIN OF 20KTS OR MORE BELOW 2000 FT AGL."
• In the freezing level GAIRMETs there are usually defined altitudes.