DOC: Viewing and editing metadata dict

[VeckoTheGecko]

The metadata for the simulation is stored in a dictionary (ParticleFile.metadata) which is passed to xarray during writing.

Parcels/parcels/particlefile.py

Lines 78 to 84 in b007dcf

	self.metadata = {
	"feature_type": "trajectory",
	"Conventions": "CF-1.6/CF-1.7",
	"ncei_template_version": "NCEI_NetCDF_Trajectory_Template_v2.0",
	"parcels_version": parcels.__version__,
	"parcels_mesh": self.parcels_mesh,
	}

Currently users can modify and add items to this dictionary, however this isn't mentioned in documentation. Showing how users can view and modify these attributes would be helpful as a quick aside in a tutorial.

NOTE: When documenting this, please use normal dictionary methods for modifying the metadata (the ParticleFile.add_metadata() method is over-engineered and will be marked for deprecation)

@erikvansebille in which tutorial would be a good place to mention this?

[erikvansebille]

Good point to better document this feature. Places to do that could be

1. The Working with Parcels Output tutorial
2. The Executing and Output section of the Parcels structure overview tutorial

If we add it in option 1, we could link it in option 2.