add some defaults values

GravityField/realfield.f90

@@ -9,12 +9,13 @@ module realfield
 
 type massbody
   real                  :: posx, posy
-  real                  :: mass
-  integer               :: serial
+  real                  :: mass          = 1.0
+  integer               :: serial        = 666
 end type
 
 !-----------------------------------------------------------------------
 contains
+!-----------------------------------------------------------------------
 
 function compute_gravity(ix, iy, body)