Issue #99: Add label_type option to Express API draw()

[hagberg]

Adds label_type ('default', 'label', 'index') to PGraph.draw() and planarity.draw(). 'index' labels vertices with the numeric indexes used by ascii(); other values raise ValueError. Includes tests and an example.

Addresses #99

Type of change

Please check only relevant options:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Changes

Added

Updated

• planarity/express/planarity.pyx - PGraph.draw() gains a label_type parameter (default 'default'). 'default' and 'label' show the original vertex labels as before; 'index' shows the numeric vertex indexes.
• planarity/express/planarity_functions.py - planarity.draw() accepts label_type.
• tests/test_express_api_draw_labels.py - Tests for each label_type value at both the function and method level.

Testing

• Full test suite passes locally (pytest -vv), including the new label_type tests

[john-boyer-phd]

Thank you for adding this Aric, it's delightful to have the flexibility to get an image version of the textual rendition. Although I accidentally omitted mentioning it in the issue, I added generating this new image variant to the express API example draw.py (and I spruced up what it says when run on the command line). I also replaced your label_type docstring in planarity_functions.py with your better version that was in planarity.pyx. I added a co-authored-by line because it was your docstring that I copied over ^_^