Allow EuclideanClusterExtraction to work on input indices

This allows for a pipeline which outputs Clusters as sets of indices
of the _original_ (organized) input cloud from the depth image camera.
That way 2d bounding boxes in the input image can be more efficiently computed.
Merge pull request #33 from v4hn/euclidean-cluster-extraction-indices

Allow EuclideanClusterExtraction to work on input indices