Add GPU Hitfinder to STS local reconstruction.

Good things come to those who wait...

This MR introduces an alternative implementation of the STS reconstruction that is runnable on GPU. It's possible to switch between both implementation via a new flag in the run_reco.C macro.

There are however still a number of limitations / issues:

• Digis for sts clusters are not collected.
• Implementation is not yet part of the new online scheme.
• Track finding doesn't work with the produced hits. Despite the hits looking correctly.

CC: @redelbach_AT_compeng.uni-frankfurt.de

reco/detectors/sts/CMakeLists.txt

@@ -122,4 +122,9 @@ include_directories(SYSTEM ${SYSTEM_INCLUDE_DIRECTORIES})
 link_directories( ${LINK_DIRECTORIES})
 GENERATE_LIBRARY()
 xpu_attach(CbmRecoSts ${DEVICE_SRCS})
+
+# FIXME: Why is this needed to install headers in subfolder?
+install(DIRECTORY experimental/ TYPE INCLUDE
+    FILES_MATCHING PATTERN "*.h"
+)
 # ---------------------------------------------------------