Code: divide charge_mixing.cpp into some files (#5427)

* divide charge_mixing.cpp into some files

* fix compile error in CI test

* add const in allocate_mixing_dmr

* initialize pointer with null

* initialize the variables and use doxygen-style notes

Author: WHUweiqingzhou

source/Makefile.Objects

@@ -630,6 +630,11 @@ OBJS_SRCPW=H_Ewald_pw.o\
     charge_mpi.o\
     charge_extra.o\
     charge_mixing.o\
+    charge_mixing_dmr.o\
+    charge_mixing_residual.o\
+    charge_mixing_preconditioner.o\
+    charge_mixing_rho.o\
+    charge_mixing_uspp.o\
     fp_energy.o\
     forces.o\
     forces_us.o\

source/module_elecstate/CMakeLists.txt

@@ -21,6 +21,11 @@ list(APPEND objects
     module_charge/charge_mpi.cpp
     module_charge/charge_extra.cpp
     module_charge/charge_mixing.cpp
+    module_charge/charge_mixing_dmr.cpp
+    module_charge/charge_mixing_residual.cpp
+    module_charge/charge_mixing_preconditioner.cpp
+    module_charge/charge_mixing_rho.cpp
+    module_charge/charge_mixing_uspp.cpp
     module_charge/symmetry_rho.cpp
     module_charge/symmetry_rhog.cpp
     fp_energy.cpp