Add ST_Accessibility

It remains to write the unit tests
Add unit tests for ST_Accessibility.

TODO: Refactor the Cormen graph prep to avoid duplicate code.
Update the README to include ST_Accessibility.

[ci skip]
Update README again
Add missing unit tests
Fix function signatures.

I had left out the TableArgument for output.edges.
Merge pull request #58 from agouge/enhancement/ST_Accessibility

Add ST_Accessibility