Dan's format as used during AR84-03
   for the example of the Iceland Sea section 
   
vm_data = 

          cid: 'ar8403a'              cruise ID
        sectid: 'IS'                  section ID
          u_dt: [45×542 double]       detided zonal velocity, 80 bins, 422 5-min profiles
          v_dt: [45×542 double]       detided meridional velocity
             u: [45×542 double]       zonal velocity, tides not removed
             v: [45×542 double]       meridional velocity, tides not removed
         depth: [45×542 double]       bin center depth
          dday: [1×542 double]        time in 2020 decimal days; noon on Jan 1 is 0.5
           lon: [1×542 double]        longitude in decimal degrees
           lat: [1×542 double]        latitude in decimal degrees
          sspd: [1×542 double]        ship speed 
      DTmodelZ: [1×542 double]        ocean depth from tidal model           
      DTmodelu: [1×542 double]        zonal barotropic tidal velocity        
      DTmodelv: [1×542 double]        meridional barotropic tidal velocity   
       DTmodel: 'tpxo9_atlas_v5'      name of the tidal model used          
        
 All velocity in meter/second;
 depths in meter;
 time in UTC, using decimal days as format. Decimal days are a zero-based year counter,
    so noon on January 1 is day 0.5, not 1.5
    For 2024, Sept 6 is decimal day 249.