Remove need for internal conversion in Acts SiSpacePointSeedMaker

We have a dedicated algorithm for converting Space Points from the InDet to the xAOD realms. However, as of now this is done as well inside the SiSpacePointSeedMaker. This MR removes the need for this internal conversion and ask the user to schedule the proper Space Point converter beforehand.

In order to retrieve the InDet space point from the xAOD counterpart, the space point converter attaches an element link to the xAOD object that allows for a quick retrieval.