add compatible_with_equinox

[chaoming0625]

This pull request includes several changes to the brainunit package, focusing on compatibility improvements, code cleanup, and documentation updates. The most important changes include adding a compatibility mode with Equinox, cleaning up redundant comments, and enhancing the documentation for the Unit class.

This feature enable the diffrax no longer need modified equinox and optimisix.
See https://github.com/chaoming0625/diffrax

Compatibility improvements:

• Added a global variable compat_with_equinox and a function compatible_with_equinox to enable or disable compatibility with Equinox. Updated the __pow__ method in the Quantity class to handle Equinox's internal _omega type. (brainunit/_base.py, [1] [2]

Code cleanup:

• Removed redundant comments in the fail_for_dimension_mismatch and fail_for_unit_mismatch functions. (brainunit/_base.py, [1] [2]

Documentation updates:

• Enhanced the Unit class documentation to include a mathematical representation of a unit. (brainunit/_base.py, brainunit/_base.pyL1276-R1293)
• Corrected and clarified various comments and docstrings, including removing references to "Brian" and updating the list of available constants. (brainunit/_base.py, [1] [2]; brainunit/_unit_constants.py, [3] [4]