initial structure for graph model

src/main/java/kieker/analysis/graph/Edge.java

src/main/java/kieker/analysis/graph/Element.java

src/main/java/kieker/analysis/graph/Graph.java

src/main/java/kieker/analysis/graph/Subgraph.java

src/main/java/kieker/analysis/graph/Vertex.java

src/main/java/kieker/analysis/graph/impl/EdgeImp.java

src/main/java/kieker/analysis/graph/impl/ElementImpl.java

src/main/java/kieker/analysis/graph/impl/GraphImpl.java

src/main/java/kieker/analysis/graph/impl/VertexImpl.java