UM,LAT,LON,VARNAME) interpolates the field VARNAME
    from the NetCDF file FILENAME onto positions given by date numbers NUM,
    latitudes LAT, and longitudes LON, and returns the results in X.
 
    The interpolation is linear interpolation performed with INTERP3.
    Longitude boundaries and near-polar latitudes are both accommodated.
 
    The field VARNAME must have three dimensions, latitude, longitude, and 
    time.  By default, the dimension order is expected to be (1) latitude,
    (2) longitude,