Relaxed the condition that ParticleFile has to be 'indexed' when deleting particles
Relaxed the condition that ParticleFile has to be 'indexed' when deleting particles

Since in principle ParticleFile can still be ‘array’ when particles are
only deleted (i.e. not added), the condition here is released.

An error is still thrown if new particles are tried to be written to
file

OceanPARCELS/parcels,OceanPARCELS/parcels