add 'fft' and 'linalg' shortcut in brainunit.math

[chaoming0625]

This pull request includes several updates to the brainunit project, including documentation updates, code formatting improvements, and integration enhancements. The most important changes are summarized below:

Documentation Updates:

• Updated repository links in README.md to reflect the new GitHub organization (chaobrain).
• Added a new section in README.md to describe the integration of brainunit with other projects in the unit-aware computation ecosystem.

Code Formatting Improvements:

• Improved readability by breaking long error messages into multiple lines in brainunit/_base.py.
• Removed unnecessary trailing commas and added missing line breaks in brainunit/_base.py. [1] [2]

Integration Enhancements:

• Added import statements for linalg and fft modules in brainunit/math/__init__.py. [1] [2]
• Added license headers and import statements in new files brainunit/math/fft.py and brainunit/math/linalg.py. [1] [2]

These changes improve the project's documentation, code readability, and integration with other components in the ecosystem.