export to MuSa

[ArcticSnow]

Add an export function to_musa() Netcdf should contain:

• variables: RH, Tair, Precip, WS
• cluster mapping

[ealonsogzl]

We need a few more variables

• Incident shortwave radiation (W m-2)
• Incident longwave radiation ( W m-2)
• Precipitation (Kg-2 m-2 s-1)
• Temperature (K)
• Relative Humidity (%)
• Wind speed (m s-1)
• Atmospheric pressure (Pa)