Refactor: split fs_nonlocal_tools.cpp by functionality

Split the 827-line fs_nonlocal_tools.cpp into four functionally-cohesive
files:
- fs_nonlocal_tools.cpp (core): constructor, destructor, allocate_memory,
  delete_memory, whole-class template instantiation
- fs_nonlocal_tools_vkb.cpp: cal_vkb, cal_becp, reduce_pool_becp
- fs_nonlocal_tools_stress.cpp: cal_vkb_deri_s, cal_dbecp_s, cal_stress
- fs_nonlocal_tools_force.cpp: cal_vkb_deri_f, cal_dbecp_f, save_vkb,
  revert_vkb, transfer_gcar, cal_force

Each new file uses method-level explicit instantiation for the CPU
specialization. Update CMakeLists.txt.

Build verified: make -j 30 in build_max_para_test succeeds.
Tests verified: MODULE_PW_pw_test, MODULE_PW_pwdft_soc, onsite_op_test,
dftu_pw/core/operator, deltaspin_pw/core all pass.

Author: abacus_fixer

source/source_pw/module_pwdft/CMakeLists.txt

@@ -49,6 +49,9 @@ list(APPEND objects
     soc.cpp
     elecond.cpp
     fs_nonlocal_tools.cpp
+    fs_nonlocal_tools_vkb.cpp
+    fs_nonlocal_tools_stress.cpp
+    fs_nonlocal_tools_force.cpp
     fs_kin_tools.cpp
     radial_proj.cpp
     onsite_proj.cpp