table: materialize_to_file return file (move-coop#449)

This commit updates the `Table.materialize_to_file` method to
return the path to the temp file that contains the data of the
Parsons `Table`. This allows users to track and clean up the
temp files as needed.

parsons/etl/table.py

@@ -197,6 +197,9 @@ def materialize_to_file(self, file_path=None):
             file_path: str
                 The path to the file to materialize the table to; if not specified, a temp file
                 will be created.
+        `Returns:`
+            str
+                Path to the temp file that now contains the table
         """
 
         # Load the data in batches, and "pickle" the rows to a temp file.
@@ -212,6 +215,8 @@ def materialize_to_file(self, file_path=None):
         # Load a Table from the file
         self.table = petl.frompickle(file_path)
 
+        return file_path
+
     def is_valid_table(self):
         """
         Performs some simple checks on a Table. Specifically, verifies that we have a valid petl

test/test_etl.py

@@ -82,7 +82,7 @@ def test_materialize(self):
     def test_materialize_to_file(self):
         # Simple test that materializing doesn't change the table
         tbl_materialized = Table(self.lst_dicts)
-        tbl_materialized.materialize_to_file()
+        _ = tbl_materialized.materialize_to_file()
 
         assert_matching_tables(self.tbl, tbl_materialized)