Remove INPUT parameter nbands_istate and unify the naming of real space wave function and partial charge density

[AsTonyshment]

Remove the nbands_istate parameter. Update the naming conventions for output files and refactor the implementation of the get_pchg and get_wf module.

Code Refactoring:

• Replaced the IState_Charge class with Get_pchg_lcao in source/module_io/get_pchg_lcao.cpp to align with the new parameter structure and improve readability.
• Simplified the logic for selecting electronic states by removing the nbands_istate parameter and relying solely on out_pchg.
• Removed unused parameters and redundant code blocks to streamline the implementation of the begin methods for both gamma-only and multi-k calculations.
• Replaced direct references to global variables with function parameters for improved modularity.

Documentation Updates:

• Removed the nbands_istate parameter from docs/advanced/input_files/input-main.md and updated references to use out_pchg instead, providing a more flexible and compact syntax for selecting electronic states.
• Clarified descriptions for out_pchg, out_wfc_norm, and out_wfc_re_im parameters, emphasizing their usage for state-based calculations and updating output file naming conventions.

[mohanchen]

LGTM