Fix(dftu): write dm_onsite.txt format matching read_occup_m parser

[dyzheng]

write_occup_m emits tokens like "Atom=1" (no space after '='), but read_occup_m reads with >> and strcmp against "Atom=". The mismatch causes NSCF runs reading dm_onsite.txt to fail with "WRONG IN READING LOCAL OCCUPATION NUMBER MATRIX FROM Plus_U FILE".

Add a space after '=' for Atom, L, ORBITAL, and spin tokens so the written format matches what read_occup_m expects. Verified with the examples/19_dftu/01_lcao_NiO DFT+U case: SCF converges and the subsequent NSCF step completes successfully.

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