Added graph and graph plotting functionality.
Merge branch 'master' of https://github.com/darpa-xdata/xlang
Merge branch 'master' of https://github.com/darpa-xdata/xlang
Merge branch 'master' of https://github.com/darpa-xdata/xlang
Merge branch 'master' of https://github.com/darpa-xdata/xlang
Merge branch 'master' of https://github.com/darpa-xdata/xlang
Merge branch 'master' of https://github.com/darpa-xdata/xlang
Merge branch 'master' of https://github.com/darpa-xdata/xlang
Added a SNAP importer and png writer for graphs.