Replaced the use of pointers by allocatables in derived data types whenever possible - Part 2

Description

Second merge request to replace the use of pointers by allocatables in derived data types whenever possible. This makes the code more robust and avoids the need to nullify the pointers before using then.

This MR also includes a bug fix: there was an incorrect intent in fourier_space_op_init.

News snippet

Replaced the use of pointers by allocatables in derived data types whenever possible (Part 2).

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.