Fix Bloch function basis in the qsymm wrapper

Currently, the function which finds the symmetries of kwant builders does not find all real space symmetries if the unit cell contains more than one sites, because the conversion from builder to qsymm model uses the wrong basis for the Bloch functions. The tests do not catch this, because they only have one site per unit cell. This fixes the basis choice, and adds a test.

Also pinging @dvarjas