Refactoring of FieldSet field store

[VeckoTheGecko]

Parcels/parcels/fieldset.py

Lines 162 to 197 in 595b7a9

	def add_field(self, field, name=None):
	"""Add a :class:`parcels.field.Field` object to the FieldSet.
	Parameters
	----------
	field : parcels.field.Field
	Field object to be added
	name : str
	Name of the :class:`parcels.field.Field` object to be added
	Examples
	--------
	For usage examples see the following tutorials:
	* `Nested Fields <../examples/tutorial_NestedFields.ipynb>`__
	* `Unit converters <../examples/tutorial_unitconverters.ipynb>`__ (Default value = None)
	"""
	if self.completed:
	raise RuntimeError(
	"FieldSet has already been completed. Are you trying to add a Field after you've created the ParticleSet?"
	)
	name = field.name if name is None else name
	if hasattr(self, name): # check if Field with same name already exists when adding new Field
	raise RuntimeError(f"FieldSet already has a Field with name '{name}'")
	if isinstance(field, NestedField):
	setattr(self, name, field)
	for fld in field:
	self.gridset.add_grid(fld)
	fld.fieldset = self
	else:
	setattr(self, name, field)
	self.gridset.add_grid(field)
	field.fieldset = self

I don't like the current implementation of .add_field() calling set_attr(self, name, field). Programmatically storing these fields messes with static typecheckers, and it would be clearer if this was handled another way (either a list of fields, a dictionary, or something else). We can set __getattr__ so users can still do fieldset.U

[erikvansebille]

Agree, this can be made much cleaner. Note though that this could also be touched upon / impacted by the unstructured grids development?

[VeckoTheGecko]

Ah yes, definitely would be impacted by unstructured grids. Might be worth refactoring ahead of time in prep for v4 (so that development is easier), but it's low on my to-do list