Added documentation for the python classes Point and PersistenceDiagram. Removed the __sub__ method for PersistenceDiagram. Minor renaming. Renamed BottleneckDistance to bottleneck_distance. Renamed intervals iterator for PersistenceDiagram to points. Seems in better conformance with usage.

[kinetic]
category=kinetic

[functionality]
category=functionality

[installation]
category=installation

[efficiency]
category=efficiency