Preserve reference to graph during vertex/edge iteration

This fixes issue #685.

This also exposes the non-descriptor based iteration in the documentation.
3 jobs for master in 68 minutes and 12 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #5013
amd64
job_clang_amd64

01:08:12

passed #5014
amd64
job_clang_cxx_amd64

00:34:33

failed #5012
amd64
job_gcc_amd64

00:55:53

 
Name Stage Failure
failed
job_clang_amd64 Test
build succeeded, 112 warnings.

Testing of doctests in the sources finished, look at the results in builds/generation.rst/output.txt.
<doctest default (setup code)[0]>:39: MatplotlibDeprecationWarning: Support for setting the 'text.latex.preamble' or 'pgf.preamble' rcParam to a list of strings is deprecated since 3.3 and will be removed two minor releases later; set it to a single string instead.
rcParams["text.latex.preamble"] = [#r"\usepackage{times}",
<doctest condensation_graph (setup code)[0]>:39: MatplotlibDeprecationWarning: Support for setting the 'text.latex.preamble' or 'pgf.preamble' rcParam to a list of strings is deprecated since 3.3 and will be removed two minor releases later; set it to a single string instead.
rcParams["text.latex.preamble"] = [#r"\usepackage{times}",
Cleaning up file based variables
ERROR: Job failed: exit status 1
failed
job_gcc_amd64 Test
build succeeded, 112 warnings.

Testing of doctests in the sources finished, look at the results in builds/generation.rst/output.txt.
<doctest default (setup code)[0]>:39: MatplotlibDeprecationWarning: Support for setting the 'text.latex.preamble' or 'pgf.preamble' rcParam to a list of strings is deprecated since 3.3 and will be removed two minor releases later; set it to a single string instead.
rcParams["text.latex.preamble"] = [#r"\usepackage{times}",
<doctest condensation_graph (setup code)[0]>:39: MatplotlibDeprecationWarning: Support for setting the 'text.latex.preamble' or 'pgf.preamble' rcParam to a list of strings is deprecated since 3.3 and will be removed two minor releases later; set it to a single string instead.
rcParams["text.latex.preamble"] = [#r"\usepackage{times}",
Cleaning up file based variables
ERROR: Job failed: exit status 1