JITParticle: Explicit default initialisation and bespoke printing
JITParticle: Explicit default initialisation and bespoke printing

Since the JIT data access does not use defaults, we explicitly
initialise variables in the particle constructor. We also make
grid indices `xi` and `yi` JIT-specific and customise the __repr__
function accordingly.

OceanPARCELS/parcels,OceanPARCELS/parcels