Bring back particle.ei in v4

[fluidnumerics-joe]

At the moment, v4 does not store particle.ei . This inhibits searching from a particle's last known position and could be a source of performance loss. We need to bring back particle.ei and enable passing in the particle's last known position in field.eval.

[erikvansebille]

Now that the grid search is so fast with the Morton encoding in #2158, we can wonder whether particle.ei is actually useful. As far as I know, the current implementation of spatial hash doe not depend on the particle index, right @fluidnumerics-joe?

The advantage of dropping particle.ei is that a Field evaluation doesn't need the particle as extra argument anymore, making the code simpler and cleaner

Of course, we would need to double-check performance with/without saving particle.ei before we definitively drop it?

[VeckoTheGecko]

Closed in #2211