Conversion of objects to/from other libraries

[iosonofabio]

This tries to address #237.

Done:

• Graph to/from networkx
• Graph to/from graph-tool
• Multigraphs to/from networkx
• Multigraphs to/from graph-tool

All these things include tests that pass locally. All tests on networkx pass on Travis too, but graph-tool is a pain to install there so that is not tested on CI.

This is ready for merging now

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

[ntamas]

FYI, this PR needs to be adjusted for the new directory structure (igraph moved under src).

[iosonofabio]

I now adjusted the PR as requested and pulled in a fresh version of the master branch. I am puzzled by the failure on AppVeyor though, it doesn't appear to be obviously related to this particular PR?

[ntamas]

Yes, that's unrelated, it is also present on the master branch. I have to look into this at some point, I just haven't found the time to do so yet.

[ntamas]

Thanks a lot! The CI tests should now pass as I fixed the MSVC build error in master.