fix(io): add space separators in out_mat_l format string

[mintleaf84]

Reminder

• Have you linked an issue with this pull request?
• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

Linked Issue

Fix #6907

Unit Tests and/or Case Tests for my changes

No additional tests required. This is a simple format string fix.

What's changed?

• Add space separators between integer fields in out_mat_l format string
• When the number of atoms exceeds 150, the columns in Lx/Ly/Lz.dat output files become indistinguishable because the %4d format fields are not separated by spaces

Any changes of core modules? (ignore if not applicable)

No changes to core modules. The modification is only in the output format string.

[mohanchen]

Thanks for your contribution!