Define named constant `min_inters`

Instead of using a literal constant.

Overlap/overlap.f90

@@ -47,6 +47,7 @@ contains
     integer l, n_select
     type(polyline) polyline_1, polyline_2
     type(polygon) res_pol
+    real, parameter:: min_inters = 0.25 ! minimum intersection between contours
 
     integer, allocatable:: selection(:)
     ! identifying numbers of a selection of extrema
@@ -111,7 +112,7 @@ contains
 
              if (res_pol%nparts /= 0) then
                 ! polyline_1 and polyline_2 overlap
-                if (spher_polygon_area(res_pol) >= 0.25 &
+                if (spher_polygon_area(res_pol) >= min_inters &
                      * min(abs(spher_polyline_area(polyline_1)), &
                      abs(spher_polyline_area(polyline_2)))) then
                    write(unit_edge, fmt = *) (k - delta) * e_overestim + i1, &