Update default value of particlefile chunks to length of pset

This fixes a bug where zarr output file writing can be _very_ slow when only one particle is released on the first day of simulation (since the default chunksize was set to the number of particles to write the first time, instead of the length of the ParticleSet)

Author: erikvansebille

parcels/particlefile.py

@@ -304,7 +304,7 @@ def write(self, pset, time: float | timedelta | np.timedelta64 | None, indices=N
 
             if self.create_new_zarrfile:
                 if self.chunks is None:
-                    self._chunks = (len(ids), 1)
+                    self._chunks = (len(pset), 1)
                 if pset._repeatpclass is not None and self.chunks[0] < 1e4:  # type: ignore[index]
                     warnings.warn(
                         f"ParticleFile chunks are set to {self.chunks}, but this may lead to "