Use intermediate constant `FACTOR`

To avoid repeated computation.

Trajectories/cost_function.py

@@ -28,6 +28,7 @@ import util_eddies
 
 Omega = 2 * math.pi / 86164.0  # in s-1
 r_Earth = 6371  # radius of the Earth, in km
+FACTOR = 2 * Omega
 
 
 def avg_radii_rossby(latitude_list, speed_list, radius_list):
@@ -43,7 +44,7 @@ def avg_radii_rossby(latitude_list, speed_list, radius_list):
     n_valid_Rossby = 0
 
     for latitude, speed, radius in zip(latitude_list, speed_list, radius_list):
-        f = 2 * Omega * math.sin(latitude)  # in s-1
+        f = FACTOR * math.sin(latitude)  # in s-1
         radius = radius * 1000  # in m
 
         if abs(speed) < 100: