v1.8.0

ControlSystems v1.8.0

Diff since v1.7.4

Closed issues:

• ssdata & storage format of systems? (#857)

Merged pull requests:

• make schur factorization differentiable (#849) (@baggepinnen)
• fix typos (#858) (@spaette)
• add linearization utility (#859) (@baggepinnen)
• Always draw up to Nyquist frequency for discrete systems (#860) (@baggepinnen)
• add option to design observers with direct term (#861) (@baggepinnen)
• add discretization of cost and covariance matrices (#862) (@baggepinnen)