Remove depth dimension in trajectory output for 2D advection

[sruehs]

It appears that per default parcels records and outputs the particle depths even for pure 2D advection (e.g., when applying the AdvectionRK4 Kernel). In my opinion, this unnecessarily enlarges the output file size. Maybe worth changing the default settings or providing an example how to tweak one's custom code to circumvent this issue?

[erikvansebille]

I dug a bit into the code, and we do have a function for this already (implemented by @CKehl):

pset.set_variable_write_status(var, write_status)

So to write only the starting depth of the particles (not at every timestep), you could do

pset.set_variable_write_status('depth', 'once')

And to not write at all, you could do

pset.set_variable_write_status('depth', False)