release 3.1 updates

CHANGELOG.md

CITATION.cff

CITATION.md

CODE

CONTRIBUTORS.md

COPYING

DEMOS/Bilinear/bilinear_BT_rail.m

DEMOS/DAE1/LQR_DAE1.m

DEMOS/DAE1/bt_mor_DAE1_tol.m

DEMOS/DAE2/IRKA_mor_Stokes.m

DEMOS/DAE2/LQR_DAE2.m

DEMOS/DAE2/bt_mor_DAE2.m

DEMOS/DAE2/private/closed_step.m

DEMOS/DAE2/private/impeuler.m

DEMOS/DAE2/private/impeuler_closed.m

DEMOS/DAE2/private/open_step.m

DEMOS/DAE2/private/smoother.m

DEMOS/DAE2_SO/LQR_DAE2_SO.m

DEMOS/DAE3_SO/BT_DAE3_SO.m

DEMOS/DAE3_SO/LQR_DAE3_SO.m