fix bug in phs_symmetrization using square root method
This commit fixes the bug in phs_symmetrization when it fails to produce ph eigenstates with PH^2 = +1 when one of the wave functions is mapped exactly onto another one by PH, as in this case the PH symmetrized wf's from the two are identical. This is the case with wfs = np.eye(2) and particle_hole = sigma, the second vector returned is an eigenstate with eigenvalue -1 not +1. In general this can be even worse, if there are multiple such vector pairs some vectors may not be ph eigenstates at all. The new code uses an improved version of the matrix square root method from Applied Mathematics and Computation 234 (2014) 380-384. Tests are modified to clear up the random unitary generation process (using kwant.rmt). Tests are now guaranteed to feed orthonormal set of vectors to phs_symmetrization(). Add extra tests for the fully off-diagonal case where the old method fails.