Philippe 21/07/2021: add missing metadata for flyers

src/MNH/write_aircraft_balloon.f90

@@ -873,12 +873,12 @@ tzbudiachro%clevels  (NLVL_SHAPE)       = 'Point'
 tzbudiachro%ccomments(NLVL_SHAPE)       = 'Values at position of flyer ' // Trim( tpflyer%title )
 
 tzbudiachro%lleveluse(NLVL_TIMEAVG)     = .false.
-tzbudiachro%clevels  (NLVL_TIMEAVG)     = ''
-tzbudiachro%ccomments(NLVL_TIMEAVG)     = ''
+tzbudiachro%clevels  (NLVL_TIMEAVG)     = 'Not_time_averaged'
+tzbudiachro%ccomments(NLVL_TIMEAVG)     = 'Values are not time averaged'
 
 tzbudiachro%lleveluse(NLVL_NORM)        = .false.
-tzbudiachro%clevels  (NLVL_NORM)        = ''
-tzbudiachro%ccomments(NLVL_NORM)        = ''
+tzbudiachro%clevels  (NLVL_NORM)        = 'Not_normalized'
+tzbudiachro%ccomments(NLVL_NORM)        = 'Values are not normalized'
 
 tzbudiachro%lleveluse(NLVL_MASK)        = .false.
 tzbudiachro%clevels  (NLVL_MASK)        = ''