Refactor: spilt `Charge_Mixing::set_mixing()` into `set_mixing()` and `init_mixing()`

[WHUweiqingzhou]

Describe the Code Quality Issue

Currently, Charge_Mixing::set_mixing() does three things:

1. pass the mixing parameters.
2. construct a mixing item.
3. allocate the memory for mixing_data.

Usually, we need to restart the mixing without updating the mixing parameters.
It is better to define a new function init_mixing() to do 2&3.

This is a step for refactor of Charge_Mixing class #3363.

Additional Context

No response

Task list for Issue attackers (only for developers)

• Identify the specific code file or section with the code quality issue.
• Investigate the issue and determine the root cause.
• Research best practices and potential solutions for the identified issue.
• Refactor the code to improve code quality, following the suggested solution.
• Ensure the refactored code adheres to the project's coding standards.
• Test the refactored code to ensure it functions as expected.
• Update any relevant documentation, if necessary.
• Submit a pull request with the refactored code and a description of the changes made.