Implement output trajectory merge in Cp2kBaseWorkChain

[yakutovicha]

ToDo:

• Add a unit test for the merge_trajectory_data function
• Update example_base_md_reftraj_restart.py so it also checks the trajectory merge.
• Implement merging logic to avoid frames with the same ids.

[cpignedoli]

OK from my side, I made a small modification to the example. I tested with the bugged aiida-core so just 1 restart.
Next PR has to be the addiiton of the array of energies to the trajectory

[cpignedoli]

tested on aiida 2.4.3