[BUG] Test code should not assume networkx is installed

**Describe the bug**
The test suite assumes that networkx is installed, although networkx is not a mandatory dependency for pygmo.

**To Reproduce**
Install pygmo and run the test suite **without** installing networkx.

**Expected behavior**
The test suite should just skip the tests using networkx, if networkx is not installed.

Start by running the test suite without networkx and identify the tests that import or use it. The work is done when those tests are skipped cleanly without networkx while the rest of the suite still runs.

Written by the indexing model from the issue text.