Combine particle.xi, particle.yi, particle.zi, particle.ti into one flattened grid_index

[erikvansebille]

During discussions about unstructured grid support for Parcels v4, we realised that it is failrly trivial to combine the particle index positions (particle.xi, particle.yi, particle.zi and particle.ti) into one particle.grid_index(?), which can then be unraveled with modulo and division operators based on xdim, ydim, zdim and tdim.

The advantage would be that the memory footprint of the ParticleSet would be less, because it would save three Variables of N particles.

[fluidnumerics-joe]

Does this duplicate (or is this duplicated by #1874 ?

[erikvansebille]

Yes indeed, I wrote this Issue quickly during our hackathon; it's the same as #1874

[VeckoTheGecko]

Duplicate of #1874 . Let's continue discussion there