Objects: Fix Missing Streamers for Signed Integer Coordinates
This fixes an oversight from !606 (merged).
I have forgotten to add dictionary definitions for signed-integer Cartesian coordinates, leading to missing streamer definitions:
Warning in <TStreamerInfo::Build>: allpix::Pixel: ROOT::Math::DisplacementVector2D<ROOT::Math::Cartesian2D<int>,ROOT::Math::DefaultCoordinateSystemTag> has no streamer or dictionary, data member "index_" will not be saved
Warning in <TStreamerInfo::Build>: ROOT::Math::DisplacementVector2D<ROOT::Math::Cartesian2D<int>,ROOT::Math::DefaultCoordinateSystemTag>: ROOT::Math::Cartesian2D<int> has no streamer or dictionary, data member "fCoordinates" will not be saved
so the data member was not stored in output files.