Refactor istate_charge.cpp to reduce dependence on other modules

[AsTonyshment]

Linked Issue

Fix #4085.

What's changed?

This pull request introduces a series of refactoring and improvements aimed at enhancing code readability, performance, and modularity. Here are the major changes made:

1. Used parse_expression Template Function:

   • Overloaded the parse_expression template function to support direct parsing to int types. This update eliminates the need for explicit type conversions previously present, reducing potential type conversion errors.

2. Isolation of bands_to_print to out_band_kb Conversion:

   • Extracted the conversion from bands_to_print to out_band_kb into a separate function. This change ensures that the process of obtaining out_band_kb is handled internally within the INPUT class, making the intermediate transformation process invisible and inaccessible from outside.

3. Reduced Dependency on pelec:

   • Modified the system to allow passing wg directly into functions where pelec was previously required. This reduction in dependency decreases coupling across the codebase.

4. Extended Functionality in efermi Struct:

   • Added a new member function, get_all_ef, to the efermi struct, which returns a vector of Fermi energies for all spins. This enhancement provides a more flexible and straightforward way to access all Fermi energy values across different spins.

5. Expansion of rhopw Parameters:

   • Decomposed the rhopw parameter into six distinct input parameters.

6. Expanded bigpw Parameters:

   • Expanded the bigpw parameter into two separate input parameters.

Reminder

• The decision to maintain the dependency on Local Orbital Charge (LOC) is due to the extensive use of its numerous member functions, which are currently tightly integrated into various parts of the codebase. Detaching this dependency now would be inefficient and could potentially destabilize critical functionalities.

Future Plans for Refactoring idmatrix:

• See Feature: Support calculation of multi-k point band-decomposed charge density under LCAO basis. #3572.

[mohanchen]

LGTM