Robust and efficient index search in Parcels

[delandmeterp]

The search of the particle index within a grid is neither robust nor efficient.
To improve it, we will need to make use of trees.
In this example, fbriol uses its pyindex module that suits our needs.

An example of using pyindex in the context of Parcels is implemented in this notebook.

To follow pyindex development, see this issue.

[erikvansebille]

This has now been implemented by @angus-g in #954