Move the asynchronous mode components into AdePT (#325)

This PR integrates the asynchronous mode components from `AsyncExample`
into AdePT as a new backend that can be activated via a compile-time
option.

In order to compile either backend into the final library
`AdePTTrackingManager.hh` includes either `AdePTTransport.h` or
`AsyncAdePTTransport.hh`, which will pull in the gcc-compiled parts of
the code. `AdePTTrackingManager.cu` includes `AdePTTransport.cuh` or
`AsyncAdePTTransport.cuh` for the nvcc-compiled side. This is done via
the cmake option `-DASYNC_MODE=[ON|OFF]`

There are no changes to the current synchronous backend, and as expected
validation shows no difference in performance or results compared to the
current `master`. For the asynchronous backend, there is no performance
difference with the current standalone example, and the validation shows
good agreement with Geant4.

<img
src="https://github.com/user-attachments/assets/a088f1d1-cad6-4669-b293-817561db8e6c"
width="70%"/>

---------

Co-authored-by: SeverinDiederichs <[email protected]>

CMakeLists.txt

@@ -165,11 +165,11 @@ endif()
 set(ADEPT_G4_INTEGRATION_SRCS
   src/G4HepEmTrackingManagerSpecialized.cc
   src/AdePTTrackingManager.cc
-  src/AdePTTrackingManager.cu
   src/AdePTPhysics.cc
   src/HepEMPhysics.cc
   src/AdePTGeant4Integration.cpp
   src/AdePTConfigurationMessenger.cc
+  src/AdePTTrackingManager.cu
 )
 
 add_library(CopCore INTERFACE)
@@ -187,6 +187,7 @@ target_include_directories(AdePT_G4_integration
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
     $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/>
 )
+
 target_link_libraries(AdePT_G4_integration
   PUBLIC
     CopCore
@@ -207,6 +208,14 @@ set_target_properties(AdePT_G4_integration
     CUDA_RESOLVE_DEVICE_SYMBOLS ON
 )
 
+option(ASYNC_MODE OFF "Enable the async transport backend")
+if(ASYNC_MODE)
+  add_compile_definitions(ASYNC_MODE)
+  message(STATUS "${Green}Using the asynchronous transport backend${ColorReset}")
+else()
+  message(STATUS "${Red}Async backend is disabled${ColorReset}")
+endif()
+
 # Optional library to activate NVTX annotations for profiling:
 option(NVTX OFF "Add NVTX instrumentation for profiling (only for annotated examples)")
 add_library(NVTX INTERFACE)