alias for unit processing functions

[chaoming0625]

This pull request includes several changes to the brainunit package, focusing on organizing imports, updating documentation, and adding a new module. The most important changes include the addition of the _alias module, reorganization of imports, and updates to the documentation structure.

New module addition:

• brainunit/math/_alias.py: Added a new module _alias with relevant imports and functions.

Reorganization of imports:

• brainunit/math/__init__.py: Added imports from the new _alias module and updated the __all__ list to include _alias_all. [1] [2] [3]
• brainunit/_base.py: Moved DIMENSIONLESS and UNITLESS to the errors section and adjusted the order of helper functions. [1] [2]

Documentation updates:

• docs/apis/brainunit.rst: Updated section titles and moved UnitMismatchError and DimensionMismatchError to the new "Errors" section. [1] [2]
• docs/auto_generater.py: Added _alias to the module_and_name list for documentation generation.