Keep original Paths objects in .score() method

The [.score](https://github.com/rdf-ext/grapoi/blob/0a6e7e99c02480e9d219194d4483a15b073abbd0/Grapoi.js#L134) method returns a `Grapoi` object with new `Path` objects. The original ones should be kept. A map is required to match the result of the [sortByScore](https://github.com/rdf-ext/grapoi/blob/0a6e7e99c02480e9d219194d4483a15b073abbd0/Grapoi.js#L132) with the original `Path` object.