Make the function `convert_id` autonomous

Not dependant on global variables. `date_num` is no longer relevant.

Convert_Matlab/convert_id.py

@@ -8,11 +8,11 @@ def date_index(date_num):
     date = datetime.date.fromordinal(date_num - 366) - datetime.date(1950, 1, 1)
     return date.days
 
-def convert_id(node_index):
+def convert_id(node_index, e_overestim, d1):
     """Convert from node identification in graph to eddy identification in
     shapefiles.
 
     """
     k = (node_index - 1) // e_overestim
     eddy_index = node_index - k * e_overestim
-    return date_index(date_num[k]), eddy_index
+    return d1 + k, eddy_index