Embed bottom and middle spacepoint in triplet

Right now, we store only the top spacepoint in the device triplet which
is technically all that is required, but this makes it very difficult
and time-consuming to retrieve the bottom and middle spacepoint. This
commit embeds the locations of those spacepoints in the triplet, making
life easier for a lot of the planned seeding changes.

Author: stephenswat

device/common/include/traccc/edm/device/device_triplet.hpp

@@ -17,7 +17,7 @@ namespace traccc::device {
 /// Triplets of bottom, middle and top spacepoints
 struct device_triplet {
     // top spacepoint location in internal spacepoint container
-    sp_location spT;
+    sp_location spB, spM, spT;
 
     using link_type = device::triplet_counter_collection_types::host::size_type;
     /// Link to triplet counter where the middle and bottom spacepoints are

device/common/include/traccc/seeding/device/impl/find_triplets.ipp

@@ -110,7 +110,7 @@ inline void find_triplets(
 
             // Add triplet to jagged vector
             triplets.at(posTriplets++) = device_triplet(
-                {spT_loc, globalIndex, curvature,
+                {spB_loc, spM_loc, spT_loc, globalIndex, curvature,
                  -impact_parameter * filter_config.impactWeightFactor,
                  lb.Zo()});
         }