Consistent naming/API

This is mainly for convenience and beauty: In the olden days of obstacles in InPartS, things like DiskObstacle and RodObstacle had roughly consistent naming and API, but it seems Disk was recently developed within this package, whereas the miscobstacles were mostly copied over from InPartS. We should decide whether we want to use the Obstacle suffix in the name and have consistent constructors for all of them. Given that we also have DiskCells, I think DiskObstacle wasn't so bad...

(#1 is related of course)