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

parcels/parcels/fieldset.py

Lines 334 to 338 in daf05b7

	indices :
	Optional dictionary of indices for each dimension
	to read from file(s), to allow for reading of subset of data.
	Default is to read the full extent of each dimension.
	Note that negative indices are not allowed.

and an example at

parcels/docs/examples/example_globcurrent.py

Lines 47 to 48 in daf05b7

	indices = {'lon': [5], 'lat': range(20, 30)}
	fieldsetsub = set_globcurrent_fieldset(indices=indices, use_xarray=use_xarray)