Introducing variables for layout queries:

1.) The ExpressionContext class is a mapping of tl::Eval
    and allows providing a variable context for the LQ.
    Expression class is derived from ExpressionContext now.
2.) The variable lookup has been changed so that variables
    can be modified even if they come from a parent context.
3.) LayoutQuery and iterator has been given an argument to
    supply the context

testdata/python/tlTest.py

@@ -24,6 +24,11 @@ class TLTest(unittest.TestCase):
 
   def test_1(self):
 
+    ctx = pya.ExpressionContext()
+    self.assertEqual(ctx.eval("1+2"), 3)
+    ctx.var("a", 21)
+    self.assertEqual(ctx.eval("2*a"), 42)
+
     expr = pya.Expression()
     res = expr.eval()
     self.assertEqual(str(type(res)).replace("class", "type"), "<type 'NoneType'>")