Rename `trajectory` variable in particle file output

[VeckoTheGecko]

Now that we don't comply with the cf-conventions in the output anyways, should we consider to rename this to e.g. particleid or even pid, which is more internally logical to Parcels. I always found the name trajectory a bit strange...

Originally posted by @erikvansebille in #2583 (comment)

[VeckoTheGecko]

Opening a separate issue so that this can be handled separately.

I agree that trajectory is confusing.

Looking at the CF conventions https://cfconventions.org/cf-conventions/cf-conventions#trajectory-data I think that we could rename the variable at any time anyway - the important thing is that we have cf_role='trajectory_id'

Do you think that trajectory_id is clear enough? I think particle_id is also good. I would rather not have pid

[erikvansebille]

I'd prefer particle_id over trajectory_id, as Parcels works with the concept of Particles throughout. A trajectory is a history/time-series of particle positions, but Parcels doesn't keep track of these histories. So Parcels doesn't have the concept of a 'trajectory'; only in the output can a user create them

Why do you think pid isn't a good option?

[VeckoTheGecko]

Discussed in person - lets go for particle_id