Read coordinates from site even when position operators are commutative.
Currently when 'build_discretized' receive onsite and hoppings to build kwant template system it perform check if input expression contains position operators as symbols. Because at this moment commutative relations doesn't play any significant role it is enough to check by symbol names.
It would make sure that
tb = {(0,): sympy.symbols('x')}
builder = build_discretized(tb, 'x', verbose=True)
will work without problems.