add momentum and position operators to 'continuum' namespace

kwant/continuum/__init__.py

@@ -20,4 +20,4 @@ except ImportError:
 del sys, ExtensionUnavailable
 
 __all__ = ['discretize', 'discretize_symbolic', 'build_discretized',
-           'sympify', 'lambdify']
+           'sympify', 'lambdify', 'momentum_operators', 'position_operators']