Clean up `__repr__` for most important Parcels classes

[erikvansebille]

As part of the tidying of Parcels v4, it would be good to also check/clean-up the __repr__ methods of classes like

• FieldSet (depending as much as possible on xarray's repr methods)
• ParticleSet (also giving access to individual particle locations? or only information on the class properties?)
• ParticleFile

This would help debugging when users start to actively work with Parcels, as they can then more easily provide information about their run

This tracking issue keeps track of the progress on this development