Use the insert() method to load files

In this way, checks are performed automatically on every tuple
and code is simplified.

This should make it safer to raise exceptions on files that are
not relations.

relational/relation.py

@@ -22,6 +22,7 @@
 
 import csv
 from itertools import chain, repeat
+from collections import deque
 
 from relational.rtypes import *
 
@@ -47,19 +48,14 @@ class relation (object):
             self.content = set()
             self.header = header([])
             return
-        # Opening file
-        fp = open(filename)
+        with open(filename) as fp:
+            reader = csv.reader(fp)  # Creating a csv reader
+            self.header = header(next(reader))  # read 1st line
+            self.content = set()
+            attributes = len(self.header)
 
-        reader = csv.reader(fp)  # Creating a csv reader
-        self.header = header(next(reader))  # read 1st line
-        self.content = set()
-
-        for i in reader:  # Iterating rows
-
-            self.content.add(tuple(map (rstring, i)))
-
-        # Closing file
-        fp.close()
+            iterator = ((self.insert(i) for i in reader))
+            deque(iterator, maxlen=0)
 
     def _make_writable(self):
         '''If this relation is marked as readonly, this