Riccati: Add option to use scipy.linalg.solve_discrete_are

[oyamad]

Close #360

[coveralls]

Coverage increased (+0.02%) to 94.894% when pulling 6858018 on oyamad:riccati-scipy into 6b1f9a8 on QuantEcon:master.

[oyamad]

In future PRs, we may add a method option to the following as well:

• In robustlq.py:

  • RBLQ.robust_rule, RBLQ.F_to_K, RBLQ.K_to_F which call lq.stationary_values

• In kalman.py:

  • Kalman.stationary_values which calls solve_discrete_riccati
  • Kalman.whitener_lss, Kalman.stationary_coefficients, Kalman.stationary_innovation_covar which call Kalman.stationary_values

[mmcky]

Thanks @oyamad for this addition -- This looks ready to merge.
I will leave open one more day for any other comment.