Fixes #928

The existing implementation of `.gexf` serialization/deserialization exhibited several issues with reading and writing the graph's `mode` attribute, leading to dynamic graphs always being interpreted as static graphs.  This fix addresses these issues.  I have removed the `mode` parameter that was being passed to `GEXFWriter`'s constructor as the mode seems like an attribute that should be stored on the graph itself (which it now is) and `write_gexf` was not passing through a value for `mode` to the constructor anyway.  

There were several cases where data attributes pertaining to the mode of the graph were being defined but not used, which I've replaced with instance variables.