``linalg.eig`` and ``linalg.eigvals`` now always return complex arrays

linalg.eig and linalg.eigvals now always return complex arrays

Previously, the return values depended on whether the eigenvalues happen to lie on the real line (which, for a general, non-symmetric matrix, is not guaranteed).

The change makes consistent what was a value-dependent result. To retain the previous behavior, do::

w = eigvals(a) if np.any(w.imag == 0): # this is what NumPy used to do w = w.real

If your matrix is symmetrix/hermitian, use eigh and eigvalsh instead of eig and eigvals. These are guaranteed to return real values. A common case is covariance matrices, which are symmetric and positive definite by construction.