do not call "sympy.expand" on the output of "continuum.sympify"

Calling "expand" at this point is not required and differs from
sympy behaviour without a good argument for it. Closes #146.

kwant/continuum/_common.py

@@ -183,7 +183,7 @@ def sympify(expr, locals=None):
             converter[list] = stored_value
         else:
             del converter[list]
-    return sympy.expand(hamiltonian)
+    return hamiltonian
 
 
 def make_commutative(expr, *symbols):

kwant/continuum/discretizer.py

@@ -370,7 +370,7 @@ def _discretize_summand(summand, coords):
         else:
             result = factor * result
 
-    return sympy.expand(result)
+    return result
 
 
 def _discretize_expression(expression, coords):