Slicing `ParticleSet` object

[VeckoTheGecko]

Currently indexing a particleset doesn't work with slice objects.

pset[0]
- > gives first particle

pset[:7]

self = <parcels.particledata.ParticleData object at 0x1620e3110>, index = slice(None, 7, None)

    def get_single_by_index(self, index):
        """Get a particle object from the ParticleData instance based on its index."""
>       assert type(index) in [
            int,
            np.int32,
            np.intp,
        ], f"Trying to get a particle by index, but index {index} is not a 32-bit integer - invalid operation."
E       AssertionError: Trying to get a particle by index, but index slice(None, 7, None) is not a 32-bit integer - invalid operation.

It would be a nice quality of life change for pset to support slicing