Workaround for a compiler bug in gcc 12

Description

It seems gcc 12 introduced a bug for polymorphic arrays and openmp

By adding a pointer, we can circumvent this and still make the code work as before.

News snippet

Add here a one line description to be included in the Changes page. You can delete this section if the changes are trivial and of no consequence for the users.

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.