Implement rank internals for the hospital-resident assignment problem

[daffidwilde]

Make the move to rank internals for the HR game.

These are the big steps to come:

• point SA at the old/current HR implementation
• rebuild the HospitalResident class and tests
• migrate algorithms into class (and their tests)
• update documentation

I'll save the matching stability and validity checks for another PR; still deciding how best to implement that...