Fix code review issues: missing imports, duplicate lock, and MPI file I/O

[Copilot]

Addresses code review feedback on PR #566 (QMC runtime variables implementation).

Changes

• Added module-level imports: output_unit and error_unit from iso_fortran_env in QMC_runtime_var_mod.F90 to fix compilation errors where symbols were used without import

• Removed duplicate lock: Eliminated redundant lock_QMC_runtime_settings() call in main.F90:485 since it's already called inside check_update_schemes_compatibility():234

• Fixed MPI file I/O: Wrapped file operations in read_and_broadcast_TEMPERING_var() with rank 0 check to prevent multiple processes from reading the same file

CALL MPI_COMM_RANK(MPI_COMM_WORLD, irank, ierr)
IF (irank == 0) THEN
  OPEN(UNIT=5,FILE='parameters',STATUS='old',ACTION='read',IOSTAT=ierr)
  ! ... read and close ...
END IF
CALL MPI_BCAST(...)  ! Broadcast to all ranks

• Minor fixes: Corrected spelling ("Bradcast" → "Broadcast") and indentation in error handling

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