Introduce a System class in the core (#4741)

Fixes #4615

Description of changes:
- completely rewrite the `EspressoSystemInterface` class
   - the new `System` class uses composition for GPU particle data management, resource deallocation, and globals
   - GPU particle data management now uses standard `thrust` vectors and properly deallocates device memory
   - simplify the `espressomd.system.System` class
- simplify GPU code
   - 9 CUDA global variables were removed (partial fix for #2628)
   - ~300 lines of CUDA code were removed

src/core/CMakeLists.txt

@@ -30,7 +30,6 @@ add_library(
   dpd.cpp
   energy.cpp
   errorhandling.cpp
-  EspressoSystemInterface.cpp
   forcecap.cpp
   forces.cpp
   ghosts.cpp
@@ -48,11 +47,12 @@ add_library(
   rotate_system.cpp
   rotation.cpp
   Observable_stat.cpp
-  SystemInterface.cpp
   thermostat.cpp
   tuning.cpp
   virtual_sites.cpp
   exclusions.cpp
+  system/GpuParticleData.cpp
+  system/System.cpp
   PartCfg.cpp
   EspressoSystemStandAlone.cpp
   TabulatedPotential.cpp)
@@ -61,13 +61,13 @@ set_target_properties(espresso_core PROPERTIES CXX_CLANG_TIDY
                                                "${ESPRESSO_CXX_CLANG_TIDY}")
 
 if(ESPRESSO_BUILD_WITH_CUDA)
-  target_sources(espresso_core PRIVATE cuda_init.cpp cuda_interface.cpp)
+  target_sources(espresso_core PRIVATE cuda/init.cpp)
   espresso_add_gpu_library(
-    espresso_cuda SHARED cuda_common_cuda.cu cuda_init_cuda.cu
-    CudaHostAllocator.cu magnetostatics/barnes_hut_gpu_cuda.cu
+    espresso_cuda SHARED cuda/common_cuda.cu cuda/init_cuda.cu
+    cuda/CudaHostAllocator.cu magnetostatics/barnes_hut_gpu_cuda.cu
     magnetostatics/dipolar_direct_sum_gpu_cuda.cu
     electrostatics/mmm1d_gpu_cuda.cu electrostatics/p3m_gpu_cuda.cu
-    electrostatics/p3m_gpu_error_cuda.cu EspressoSystemInterface_cuda.cu)
+    electrostatics/p3m_gpu_error_cuda.cu system/GpuParticleData_cuda.cu)
   add_library(espresso::cuda ALIAS espresso_cuda)
   target_link_libraries(
     espresso_cuda PRIVATE CUDA::cuda_driver CUDA::cudart CUDA::cufft