sci-mathematics/planarity: new package of graph planarity algorithms.

sci-mathematics/planarity: new package of graph planarity algorithms.

This commit imports the "planarity" package from the sage-on-gentoo
overlay with a few minor changes:

  * Update to EAPI=7.
  * Drop autotools.eclass and use the tarball from sagemath.org
    that contains the autotools stuff.
  * Add USE=examples to install some sample input/output.
  * Update the old code.google.com HOMEPAGE to point to github.

The SageMath project now has the ability to detect and use a system
copy of planarity, so this will help Gentoo users of SageMath avoid
some pointless rebuilds of planarity.

Closes: https://bugs.gentoo.org/707324
Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>