Fix DFT+U occupation matrix output format

[lanshuyue]

Problem

write_occup_m() currently writes DFT+U occupation-matrix labels and integer values without whitespace separation:

Atom=1 L=2 ORBITAL=0
spin=1

However, read_occup_m() uses whitespace-delimited stream extraction and expects the labels and values as separate tokens:

Atom= 1 L= 2 ORBITAL= 0
spin= 1

As a result, the DFT+U band workflow cannot read the dm_onsite.txt generated by the preceding SCF calculation and fails with:

WRONG IN READING LOCAL OCCUPATION NUMBER MATRIX FROM Plus_U FILE

The regression context is fa14bddd5666ba254b02d3cbc2c83a4db6b3bb98 (DFT+U Refactor (#7538)).

Fix

This adds the missing whitespace separator between the labels and their integer values in write_occup_m().

No reader logic is changed.
No DFT+U numerical behavior is changed.
No occupation matrix values are changed.
No INPUT behavior is changed.

Validation

• Reproduced the failure using the existing NiO DFT+U SCF -> band workflow.

• Confirmed that the original writer generated compact tokens such as Atom=1 and spin=1.

• Confirmed that adding whitespace after = alone allowed the existing reader to complete the band calculation.

• Confirmed that source/source_lcao/module_dftu/dftu_io.cpp has no writer/reader diff from v3.11.0-beta8 to current upstream/develop.

• Rebuilt beta8-equivalent DFT+U code after applying this writer fix.

• Confirmed that SCF now generates the expected whitespace-separated labels and values automatically:

  Atom= 1 L= 2 ORBITAL= 0
  spin= 1
  

• Used the newly generated dm_onsite.txt directly, without manual modification.

• Confirmed that the subsequent band calculation completed and produced bands1.txt and bands2.txt.

Automated regression test added: no.
Runtime reproduction/validation: yes.

Note: the current upstream/develop full local Intel build is blocked on this workstation by an unrelated LR compile error in source/source_lcao/module_lr/utils/exciton_plotter.cpp. Runtime validation was therefore performed with the beta8-based validation worktree, whose dftu_io.cpp writer/reader code is unchanged from current upstream/develop before this fix.