Add chemical potential parameter mu to Hubbard model

[Copilot]

Adds a chemical potential mu to the Hubbard model Hamiltonian (-μΣn_{iσ}), and clarifies that electron_number is ignored by networks without particle number conservation.

Changes

• ModelConfig: New mu: float = 0 field representing chemical potential
• _prepare_hamiltonian: Adds -mu * n_{iσ} diagonal terms for each site and spin orbital
• electron_number comment: Updated to note the parameter is ignored when using networks without particle number conservation
• Logging: __init__ log message now includes mu

Example

from qmp.models.hubbard import ModelConfig, Model

# Grand canonical ensemble — tune filling via chemical potential
cfg = ModelConfig(m=4, n=4, t=1.0, u=8.0, mu=0.5)
model = Model(cfg)

Original prompt

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This section details on the original issue you should resolve

<issue_title>给hubbard模型添加化学势参数mu, 并修改 "electron_number"的注释, 说明如果使用的是没有限制粒子数的网络, 这个参数会被忽略.</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 给hubbard模型添加化学势参数mu, 并修改 "electron_number"的注释, 说明如果使用的是没有限制粒子数的网络, 这个参数会被忽略. #198

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