This release include a number of new features and bug fixes. The main
focus of the improvements of this release was to increase the ease of interacting with graph objects. This includes adding support for generating dot
output which can be used with graphviz (or similar tools) for visualizing
graphs adding more methods to query the state of graph, adding a generator
module for easily creating graphs of certain shape, and implementing the
mapping protocol so you can directly interact with graph objects.

Added
-----
- A new method, to_dot(), was added to PyGraph and PyDiGraph. It will
  generate a dot format representation of the object which can be used
  with Graphivz (or similar tooling) to generate visualizations of the
  graph.
- Added a new function, strongly_connected_components(), to get the list
  of strongly connected components of a PyDiGraph object.
- A new method, compose(), for combing another graph object of the same
  type was added to PyGraph and PyDiGraph.
- The PyGraph and PyDigraph classes now implement the Python mapping
  protocol for interacting with graph nodes. You can now access and
  interact with node data directly by using standard map access patterns
  in python. For example, running 'graph[1]' will return the data for the
  index at node 1.
- A new module, retworkx.generators, has been added. Functions in this
  module can be used for quickly generating graphs of certain shape.
- A new method, remove_node_retain_edges(), has been added to the
  PyDiGraph class. This method can be used to remove a node and add
  edges from its predecesors to its successors.
- Two new methods, edge_list() and weighted_edge_list(), for getting a
  list of tuples with the edge source and target (with or without edge
  weights) have been added to PyGraph and PyDigraph.
- A new function, cycle_basis(), for getting a list of cycles which form
  a basis for cycles of a PyGraph object.
- Two new functions, graph_floyd_warshall_numpy() and
  digraph_floyd_warshall_numpy(), were added for running the Floyd Warshall
  algorithm and returning all the shortest path lengths as a distance
  matrix.
- A new constructor method, read_edge_list(), has been added to PyGraph and
  PyDigraph. This method will take in a path to an edge list file and
  will read that file and generate a new object from the contents.
- A new method, extend_from_edge_list(), has been added to PyGraph and
  PyDiGraph. This method takes in an edge list and will add both the edges and
  nodes (if a node index used doesn't exist yet) in the list to the graph.

Fixes
-----

- The limitation with is_isomorphic() and is_isomorphic_node_match() functions
  that would cause segfaults when comparing graphs with node removals
  has been fixed. You can now run either function with any
  PyDiGraph/PyDAG objects, even if there are node removals. Fixes #27
- If an invalid node index was passed as part of the first_layer
  argument to the layers() function