Add non-overlapping option in Random Tiler

[nicolebussola]

To allow for non-overlapping tiles in the randomtiler, we could re-define the tissue mask every time a random tile is extracted, by removing the tile before finding the new coordinates.

In each iteration during the random tiles extraction we should set to True (1) the segments for the tile extracted during the current iteration in order to not extract any of the segment in the next tiles extractions.