Add tutorial information about indices keyword when creating FieldSet

[erikvansebille]

The option to use the indices dictionary specify what part of a Field to load can be useful at times, but is not discussed anywhere in the documentation. Perhaps good to add it somewhere, so that more users know about it. For example in the Grid Indexing tutorial?

See the FieldSet.from_netcdf() docstring at
https://github.com/OceanParcels/parcels/blob/daf05b71174ba2c687bfe0468ed27e1e64f17d06/parcels/fieldset.py#L334-L338 and an example at
https://github.com/OceanParcels/parcels/blob/daf05b71174ba2c687bfe0468ed27e1e64f17d06/docs/examples/example_globcurrent.py#L47-L48