[math] Add new module linalg and more numpy methods (diag_indices_from & det & norm)

[Routhleck]

This pull request introduces new functions and updates to existing ones in the brainunit library, specifically focusing on linear algebra and mathematical operations. The changes include adding new functions for computing determinants, norms, and diagonal indices, along with corresponding unit tests to ensure correctness.

New Functions:

• Added det function to compute the determinant of an array in brainunit.math (brainunit/math/_fun_change_unit.py).
• Added norm function to compute the norm of a matrix or vector in brainunit.math (brainunit/math/_fun_keep_unit.py).
• Added diag_indices_from function to return indices for accessing the main diagonal of a given array in brainunit.math (brainunit/math/_fun_remove_unit.py).

Updates to Existing Functions:

• Included det in the list of linear algebra functions in brainunit.math (brainunit/math/_fun_change_unit.py).
• Included norm in the list of mathematical functions that keep units in brainunit.math (brainunit/math/_fun_keep_unit.py).
• Included diag_indices_from in the list of indexing functions in brainunit.math (brainunit/math/_fun_remove_unit.py).

Unit Tests:

• Added tests for the det function in brainunit/math/_fun_change_unit_test.py to verify the correctness of determinant calculations with and without units.
• Added tests for the norm function in brainunit/math/_fun_keep_unit_test.py to ensure the norm calculations are accurate and maintain units when necessary.
• Added tests for the diag_indices_from function in brainunit/math/_fun_remove_unit_test.py to validate the correct indices are returned for accessing the main diagonal of arrays.

[chaoming0625]

Geat work.