This includes a spirit parser for the gml syntax, and a graph writer.

Additionaly, the general graph io code also been cleaned up.

Images are now converted to png a posteriori for html output, but are
used as pdf for latex.

This better organizes the documentation of the Graph class and provides
other small fixes.

Small fixes of some examples in the docstrings.

This completes all the documentation up to this point.

This modified several docstrings and introduces a quick-start guide.