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.

This allows pinning of individual vertices via the `pin` paramenter, as
well as string-valued color property maps, and some other minor
improvements.

Added more DOI links to references, and other cosmetic changes.

Now the meaning of the bin edges given to the several histogram
functions are the same as in numpy/scipy. This means that the last bin
edge represents an upper bound on the last bin.

This completes all the documentation up to this point.

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