[feat] Add fft module

[Routhleck]

This pull request introduces several significant changes to the brainunit package, including the addition of FFT functionality, a new hash method, and the inclusion of copyright headers in various test files.

New FFT Functionality:

• Added FFT module to brainunit with imports and updated __all__ to include fft in brainunit/__init__.py. [1] [2]
• Implemented FFT functions and tests in brainunit/fft:
  • brainunit/fft/__init__.py: Added FFT-related imports and __all__ definitions.
  • brainunit/fft/_fft_change_unit_test.py: Added tests for FFT change unit functions using parameterized tests.
  • brainunit/fft/_fft_keep_unit.py: Implemented fftshift and ifftshift functions.
  • brainunit/fft/_fft_keep_unit_test.py: Added tests for FFT keep unit functions using parameterized tests.

Enhancements to brainunit/_base.py:

• Added __hash__ method to Unit class to enable hashing based on unit attributes.

Copyright Headers:

• Added copyright headers to several test files:
  • brainunit/lax/_lax_accept_unitless_test.py
  • brainunit/lax/_lax_array_creation_test.py
  • brainunit/lax/_lax_change_unit_test.py
  • brainunit/lax/_lax_keep_unit_test.py
  • brainunit/lax/_lax_linalg_test.py
  • brainunit/lax/_lax_remove_unit_test.py
  • brainunit/lax/_misc_test.py
  • brainunit/math/_fun_keep_unit_test.py

[chaoming0625]

Great