Use bare pointers in ACTS space points

This will modify the Space Point EDM (we currently have not persistified them in any AOD so we don't care yet about schema evolution) replacing a vector of element links to clusters with bare pointers (ElementLink<xAOD::UncalibratedMeasurementContainer> -> std::vector<const xAOD::UncalibratedMeasurement*>). This resulted in a faster space point formation (quite a sensible improvement!).

When it comes to possible persistification of these objects, the strategy proposed by @akraszna was to create ad-hoc decoration algorithms that will decorate the space points with a vector of elements links to measurements. We then persistify this vector of elements links, while the vector of bare pointers is always ignored.

This is included in this MR and from some checks I've made it looks like it is working.