[#1] updated gradoop version

[taucontrib]

• Updated the Gradoop version to 0.5.1
• Changed all objects of type Vertex | Edge | GraphHead to EPGMVertex | EPGMEdge | EPGMGraphHead
• All objects of type VertexFactory belong to VertexFactory<EPGMVertex> now (same with EdgeFactory and GraphHeadFactory)
• all occurence of the GradoopFlinkConfig.getVertexFactory() been changed to GradoopFlinkConfig.getLogicalGraphFactory().getVertexFactory() or GradoopFlinkConfig.getGraphCollectionFactory.getVertexFactory() (same withEdges and GraphHeads
• the transformation from objects of type Date into type Long in the ComplaintHandling class in line 196 e.g. is now done via the PropertyValue.getDate().getLong(ChronoField.ERA) method call
• The AbstractRunner class, super class for the SyntheticDataGenerator example is not included in the examples of Gradoop 0.5.1 but in the latest Benchmark version. I don´t wanted to include the Benchmark repo as a dependency so i added the AbstractRunner class in the Gradoop-Generator project
• The whole CategoryCharacteristicsSubgraphTest is (still?) not working and gets ignored. HasLabel<>() is an aggregate function in the new Gradoop version and can´t be used as a filter function anymore
• Changed the copyright of the license headers of each class to 2019
• added a .github.workflows directory for the maven.yml file identicall to the build in Gradoop/Gradoop-Capf etc
• someone has to check/update the pom.xml script