Bugfix: td_occup with states parallelization and isolated systems

Description

Fix various problems related to the header of td_occup output for isolated systems in states parallelization.

News snippet

Fix various problems related to the header of td_occup output for isolated systems in states parallelization.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.