Harmonize naming of SSSP

[mihaic]

In graphalytics-core, it's called SingleSourceShortestPaths, e.g.: https://github.com/tudelft-atlarge/graphalytics/blob/master/graphalytics-core/src/main/java/nl/tudelft/graphalytics/domain/algorithms/SingleSourceShortestPathsParameters.java

In graphalytics-platforms-giraph and the other platforms, it's called SingleSourceShortestPath, e.g.: https://github.com/tudelft-atlarge/graphalytics-platforms-giraph/blob/master/graphalytics-platforms-giraph-platform/src/main/java/nl/tudelft/graphalytics/giraph/algorithms/sssp/SingleSourceShortestPathJob.java https://github.com/tudelft-atlarge/graphalytics-platforms-graphx/blob/master/graphalytics-platforms-graphx-platform/src/main/scala/nl/tudelft/graphalytics/graphx/sssp/SingleSourceShortestPathJob.scala