Rename shapefile center to centroid

We follow Matlab naming. We want the transition in output format to be
as easy as possible.

Convert_Matlab/inst_eddies_v6.py

@@ -27,8 +27,8 @@ def define_fields(writers):
     writers["extr"].field("eddy_index", "N", 5)
     writers["extr"].field("speed", "N", 13, 6)
 
-    writers["center"].field("days_1950", "N", 5)
-    writers["center"].field("eddy_index", "N", 5)
+    writers["centroid"].field("days_1950", "N", 5)
+    writers["centroid"].field("eddy_index", "N", 5)
 
     writers["outer"].field("r_eq_area", "N", 10, 4)
     writers["outer"].field("ssh", "N", 13, 6)
@@ -68,8 +68,8 @@ def write(writers, cell, cyclone):
         else:
             ssh =  matlab_data["ADT"][i, j] - eddy[7]
 
-        writers["center"].point(eddy[2], eddy[3])
-        writers["center"].record(days_1950 = d, eddy_index = eddy_index)
+        writers["centroid"].point(eddy[2], eddy[3])
+        writers["centroid"].record(days_1950 = d, eddy_index = eddy_index)
 
         writers["outer"].record(r_eq_area = eddy[6], ssh = ssh, days_1950 = d,
                                 eddy_index = eddy_index)
@@ -112,8 +112,9 @@ for f in factories:
     file = path.join(f["SHP_coll"], "extremum")
     f["writers"]["extr"] = shapefile.Writer(file, shapeType = shapefile.POINT)
     
-    file = path.join(f["SHP_coll"], "center")
-    f["writers"]["center"] = shapefile.Writer(file, shapeType = shapefile.POINT)
+    file = path.join(f["SHP_coll"], "centroid")
+    f["writers"]["centroid"] = shapefile.Writer(file,
+                                                shapeType = shapefile.POINT)
     
     file = path.join(f["SHP_coll"], "outermost_contour")
     f["writers"]["outer"] = shapefile.Writer(file,