Dev dm pixel spacepoint features

This update improves some small elements of the AthenaReader. Mostly this is motivated by having both strip and pixel spacepoints be a combination of two clusters. Meaning that both strip and pixel spacepoints now have two associated clusters. Although, of course, the pixels clusters have the same features (i.e. they are simply duplicated). This is to make it easier to consume cluster information with a homogeneous model.