This PR contains some small fixes that I discovered while teaching CDS 112 at Caltech in Winter 2023:

• Fixed bug in create_statefbk_iosystem that didn't proper handle 1D gain schedules.
• flatsys.point_to_point generates an error if you specify a cost or constraint with less than 3 time points.
• Sampling a LinearIOSystem returns a LinearIOSystem (before it returned a StateSpace system).
• Added an add_unused keyword parameter to interconnect that allows unused inputs or outputs to be added as inputs or outputs of the interconnected system (useful for doing a "partial" interconnection).
• create_mpc_iosystem now allows system and signal names to be specified via appropriate keywords.
• Changed type keyword for create_statefbk_iosystem to controller_type (linear or nonlinear). Using type generates a deprecation warning.
• Added control_indices and state_indices to create_statefbk_iosystem to allow partial interconnection (eg, for inner/outer loop construction).
• Updated unit tests, docstrings, and comments.

Coverage: ?%. Remained the same when pulling 10bcd65 on murrayrm:cds112_fixes-12Jan2023 into 55730b6 on python-control:main.

Coverage: 94.876% (-0.007%) from 94.883% when pulling 574cf69 on murrayrm:cds112_fixes-12Jan2023 into 08dcc95 on python-control:main.

Rebased onto current main, in order to resolve conflicts due to #857