Add default initialization for component area

Add default initialization for component area of derived type
`ssh_contour`, because we are going to keep contours for which we will
not compute area.

Common/derived_types.f90

@@ -11,7 +11,7 @@ module derived_types
      ! Points of polyline are (longitude, latitude) in rad. Polyline
      ! should be closed, except if it is null.
      real ssh ! in m
-     real area ! in m2
+     real:: area = - 1e6 ! in m2
   end type ssh_contour
 
   type extremum