Improve the separation of islands method

[JulienPercheron]

The current separation method is based on a grid and adjacent vegetation cells are considered part of the same island.
Bellow is an example of how the separation method currently works :

The algorithm implementation is correct. Yet two trees touch each other only by two extended branches the algorithm will not separate them and consider them as a single island. In other terms the limit of the current algorithm is its limited separation criteria: when are two trees apart enough to be considered a two trees ? If we could separate them, we could have better hull reconstruction method results.