Add new method to set default magnitude of Units

[Routhleck]

>>> set_default_magnitude('n') # Sets the default magnitude to 'nano' (10e-9)
>>> set_default_magnitude(-9) # Alternatively, use an integer to represent the exponent of 10
>>> set_default_magnitude({'m': -3, 'kg': -9}) # Set the default magnitude for 'metre' to 'milli' and 'kilogram' to 'nano'
>>> set_default_magnitude({'m': 'm', 'kg': 'n'}) # Alternatively, use a string to represent the magnitude

More examples at https://github.com/chaoming0625/brainunit/blob/c25506c9ff9c03dc225ba541f8c517b98a7309f3/brainunit/_unit_test.py#L1477-L1570

[chaoming0625]

Sound very great. I am checking now.

[Routhleck]

Almost all test cases need to be fixed

[Routhleck]

Need to set base

[chaoming0625]

Almost all test cases need to be fixed

这么吓人？

[Routhleck]

Almost all test cases need to be fixed

这么吓人？

https://github.com/chaoming0625/brainunit/actions/runs/10250404914/job/28356059580
262 failed, 33 passed, 2 warnings in 43.34s

[chaoming0625]

先不管tests，你先改math，改完后我再看，最后再改tests