Added grouping of TEMPO

For now this works the same as remarks, but it would probably be better
as its own class with time from, time to and a new set of weather
conditions of its own. 

Here is an example METAR from EHAM:
>EHAM 212055Z 34009KT 300V020 5000 BR FEW005 SCT006 BKN007 19/19 Q1016 TEMPO SCT007

The SCT007 at the end came up as 700 feet scattered in the cloud layers rather than a forecast component for the next two hours.