Add electron number into group name for Hubbard model.

[hzhangxyz]

Description

Checklist:

• I have read the CONTRIBUTING.md.

[Copilot]

Pull Request Overview

This PR enhances the naming of Hubbard model groups by appending the electron count to the default group name.

• Updated default_group_name to include config.electron_number
• No other functional changes

Comments suppressed due to low confidence (2)

qmb/hubbard.py:61

• Consider updating the docstring for default_group_name to document that electron_number is now part of the naming convention.

        return f"Hubbard_{config.m}x{config.n}_t{config.t}_u{config.u}_e{config.electron_number}"

qmb/hubbard.py:61

• Add or update a unit test for default_group_name to verify that the electron_number is correctly included in the output string.

        return f"Hubbard_{config.m}x{config.n}_t{config.t}_u{config.u}_e{config.electron_number}"