feat: sync DFT+U/DeltaSpin code from feat/dftu-pw-port-v2

Direct sync of source/ directory from feat/dftu-pw-port-v2 to develop.
276 files changed, +9414/-3864 lines.

Includes:
- DFT+U/DeltaSpin nspin=1/2/4 support
- sc_lambda_strategy, sc_direction_only, sc_scan_* parameters
- DFT+U occupation mixing (mixing_dftu, allocate_mixing_uom)
- Plus_U accessor functions
- npol-aware force/stress/onsite kernels
- op_pw_proj nspin=1 DeltaSpin support
- All other code modifications from port-v2

Author: dyzheng