Update the `points_association_nn` tool for compatibility with the `spot_detection_2d` tool

Since the output format of the [`spot_detection_2d`](https://github.com/BMCV/galaxy-image-analysis/tree/master/tools/points2binaryimage) tool has changed in #132, the [`points_association_nn`](https://github.com/BMCV/galaxy-image-analysis/tree/master/tools/points_association_nn) tool needs to be updated to support the new format.