Added some missing mapping functions to Body

* Body.getLocalCenter()
* Body.toWorldPoint(localPoint)
* Body.toWorldVector(localVector)
* Body.toLocalPoint(worldPoint)
* Body.toLocalVector(worldVector)

For some of these of course also `Item.mapToItem` and `Item.mapFromItem` could be used, but I don't think it hurts to have them anyway.