GeodeticPath. Be tolerant with multiple identical source & target datums

[steffen]

When we have a more than one geodetic paths with identical source & target datum be tolerant and use the first one. The forward paths are preferred over the backward ones. Note that the definitions out of the user dictionary are first and therefore have priority in this case. The identical behavior already happens with the geodetic transformations (Method CS_gxdef is used and not CS_gxdefEx). Now implement it consistent also for the paths. The use case is that a system geodetic path can be overwritten by a custom one.