support msgpack checkpoint of brainunit.Quantity

[chaoming0625]

This pull request introduces several enhancements and fixes to the braintools/file/msg_checkpoint.py file, including adding support for brainunit quantities, improving the msgpack_load function, and adding a new unit test. Below are the most important changes:

Enhancements to Serialization:

• Added support for brainunit quantities by defining _quantity_dict_state and _restore_quantity functions and registering them with register_serialization_state.

Improvements to msgpack_load Function:

• Added an optional target parameter to the msgpack_load function to allow restoring state directly into an object. Updated the function's docstring and the read_chunk function to handle this new parameter. [1] [2] [3]

Code Cleanup:

• Removed commented-out code related to array serialization.

Imports and Module Management:

• Added import for brainunit as u to support the new quantity serialization.
• Removed unnecessary blank line.

Unit Testing:

• Added a new unit test TestMsgCheckpoint in braintools/file/msg_checkpoint_test.py to validate the msgpack_save and msgpack_load functions with brainunit quantities.