Additional and last updates for version 0.4 of the openEO API

• Restructuring of graph classes and module setup. The following things changed in terms of the code:
  • renamed node.graph to node.content
  • all operations on a graph (dependencies, ancestors, lineage, ...) return now a subgraph
  • a graph has two new properties: ids and nodes. ids are the node IDs and nodes the nodes. Both are views
  • nnodes was removed and can be replaced by calling len(graph)
  • new class method from_list converts a list of nodes to a graph
  • __getitem__ method in the graph class supports indexing by integer and node ID
  • get_node_by_name method in the graph class returns the first node matching a given name
  • nodes_at_same_level in the graph class was renamed and adapted to find_siblings (all nodes having the same parent)
• Additional tests