Remove NumPy matrix class

[murrayrm]

This PR removes the use of the NumPy matrix class from python-control. The main changes are:

• The use_numpy_matrix function is no longer available.
• The use of MATLAB-style matrix strings ("[[1 2; 3 4]]") is no longer supported.
• The use_legacy_defaults function returns a warning if the version is pre-0.9 (since those versions used the np.matrix type for state space systems).
• Unit tests and CI tests were updated to remove testing against np.matrix.
• Docstrings and documentation were updated to be compatible with the above changes.

[coveralls]

coverage: 94.613% (+0.006%) from 94.607% when pulling 77a75a6 on murrayrm:remove_matrix-10Jun2023 into 71bbce2 on python-control:main.