Create intermediary variable `delta_long`

Preparing for more complex association criterium.

Overlap/overlap.f90

@@ -40,6 +40,7 @@ contains
     integer l, n_select
     type(polyline) polyline_1, polyline_2
     logical assoc_eddies ! association between eddies
+    real delta_long ! difference in longitude, in rad
 
     integer, allocatable:: selection(:)
     ! identifying numbers of a selection of extrema
@@ -91,11 +92,12 @@ contains
                  polyline_2 = e2%out_cont%polyline
               end if
 
+              delta_long = e1%extr%coord(1) - e2%extr%coord(1)
+
               ! Shift the longitudes of polyline_2 to values close to the
               ! longitude of extremum i1:
               polyline_2%points(1, :) = polyline_2%points(1, :) &
-                   + floor((e1%extr%coord(1) - e2%extr%coord(1)) / twopi &
-                   + 0.5) * twopi
+                   + floor(delta_long / twopi + 0.5) * twopi
 
               assoc_eddies = inters_lines(polyline_1, polyline_2, &
                    min_inters_speed)