fix check for sympy classes in sympify

As of version 1.1.1 Sympy does not include MutableDenseMatrix
in 'all_classes', so expressions containing such terms did not trigger
the warning in kwant.continuum.sympify when 'locals' are provided.

Closes #180.

kwant/continuum/_common.py

@@ -25,6 +25,9 @@ import warnings
 
 from .._common import reraise_warnings
 
+# TODO: remove when sympy correctly includes MutableDenseMatrix (lol).
+sympy_classes = set(sympy_classes) | {sympy.MutableDenseMatrix}
+
 momentum_operators = sympy.symbols('k_x k_y k_z', commutative=False)
 position_operators = sympy.symbols('x y z', commutative=False)