FIX: Raise correct error when A is not square in LinearStateSpace

[QBatista]

Currently:

A = np.zeros((1, 2))
C = np.zeros((1, 1))
G = np.zeros((1, 1))

qe.LinearStateSpace(A, C, G)

raises

TypeError: not all arguments converted during string formatting

This PR fixes lss.py so that the correct error is raised when A is not a square matrix.

[coveralls]

Coverage increased (+0.02%) to 93.91% when pulling fc629de on QBatista:fix_error_lss into 1e757ec on QuantEcon:master.

[oyamad]

Thanks!