Refactor: Mulliken output codes with more flexible io interfaces

[hongriTianqi]

Reminder

• Have you linked an issue with this pull request?
• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?

Linked Issue

Fix #4170
Fix #3802

Unit Tests and/or Case Tests for my changes

• Unit tests have been added for the newly added two classes: CellIndex and Output_Mulliken
• Add a new case test 214_NO_mulliken_nscf after solving issue 3802.

What's changed?

• The CellIndex class might be helpful in refactoring UnitCell as it can be used to replace GlobalC::ucell when only cell indices are needed. For example, the function ofwrite_orb_info has been added as a member function of CellIndex.
• Two classes Output_Sk and Output_DMK are added to provide an interface to get the pointers of Sk and DMK only in this work. They might be helpful in refactoring LM.

Changes in ESOLVER

• void ESolver_KS_LCAO<TK, TR>::nscf(void): The option to output Mulliken population in nscf calculation has been added to fix mulliken charge output is disabled in nscf calculation #3802
• a new function void cal_mag(const int istep, const bool print = false); is added in ESolver_KS_LCAO to be used both in scf and nscf calculation.