Move Graph degree helpers to new Algorithm\Degree

Move `Graph::getDegree()`, `Graph::getDegreeMin()`,
`Graph::getDegreeMax()` and `Graph::isRegular()` to new
`Algorithm\Degree`
Add test with 100% coverage for new Algorithm\Degree
Document new Algorithm\Degree
Move Graph::isBalanced() to Algorithm\Degree
Improve documentation for Algorithm\Degree
Merge pull request #29 from clue/move-algo-degree

Move Graph degree helpers to new Algorithm\Degree