AbstractGraphReporter.network_measures should not be abstract

[jayqi]

AbstractGraphReporter.network_measures should not be abstract. The change in this PR implements the same logic as in the R package. For now though, trying to call it will result in pkg_graph.graph_measures() raising a NotImplementedError, which is sensible.

Fixes the issue reported here: #267 (comment)

[jameslamb]

makes sense to me!

[bburns632]

LGTM. There's a failed test due to gnutls loading failure in Travis. I'd like to look into that, but if we're still looking into that by this time tomorrow, it's probably safe to merge.

[jayqi]

It might be that the Mac build (with the gnutls error) was canceled because the Linux build had a C++ compilation error when installing devtools. The builds don't really matter at this point because the py-pkg branch doesn't touch the R code, and I don't have tests for the Python package yet.

[jameslamb]

It might be that the Mac build (with the gnutls error) was canceled because the Linux build had a C++ compilation error when installing devtools. The builds don't really matter at this point because the py-pkg branch doesn't touch the R code, and I don't have tests for the Python package yet.

@jayqi I'm ok with this, you can use your admin rights to merge this one