[graph-tool] bug + fix in blockmodel.py

Hi there, I'd like to report a bug in blockmodel.py, built from source where __version__ = 2.2.28dev (commit da179ff0, Thu Dec 5 00:41:09 2013 +0100) Line 222 has return self.mrp. self.mrm this line returns a tuple of the two PropertyMaps, the full stop should be a comma :) kind regards, Alan