Prompted by issue #1100, this PR updates the _ssmatrix and _check_shape functions to make their usage more consistent and remove legacy calls that are no longer needed.

Changes:

• Updated _ssmatrix to allow checking for square matrices and row/col dimensions, with improved error messages that include the name of the matrix generating the error.
• Functions that accept state-space matrices now call _ssmatrix for uniform processing.
• Removed unneeded calls to _ssmatrix on outputs of functions in mateqn.py and other places. These calls were used back when we supported the NumPy matrix class and are no longer required.
• Fixed acker to use the matrices generated by _ssmatrix.
• Updated _check_shape to have a call signature that is more conistent with _ssmatrix: the matrix and dimensions are first, followed by optional keywords, with a name keyword for the name of the matrix being checked (for error messages).
• Removed internal _check_shape in statefbk.py and replaced with a call to mateqn._check_shape.
• Updated unit tests to reflect the new wording of some error messages.

coverage: 94.661% (+0.002%) from 94.659%
when pulling 4fc70f7 on murrayrm:update_ssmatrix-01Feb2025
into 2cb0520 on python-control:main.