move update code for postcode and word count to Python

Adds also tests for the new function to execute a SQL script.

nominatim/db/utils.py

@@ -0,0 +1,11 @@
+"""
+Helper functions for handling DB accesses.
+"""
+
+def execute_file(conn, fname):
+    """ Read an SQL file and run its contents against the given connection.
+    """
+    with fname.open('r') as fdesc:
+        sql = fdesc.read()
+    with conn.cursor() as cur:
+        cur.execute(sql)