Add force calculation kernel and fix incorrect ctest configuration for Cuda tests

kernels/CMakeLists.txt

@@ -2,12 +2,14 @@ project(${NAME}_cuda_lib CUDA CXX)
 
 set(HEADER_FILES
     pair_potentials.cuh
+    forces.cuh
 )
 set(SOURCE_FILES
+    forces.cu
 )
 
 # The library contains header and source files.
-add_library(${NAME}_cuda_lib INTERFACE
+add_library(${NAME}_cuda_lib STATIC
     ${SOURCE_FILES}
     ${HEADER_FILES}
 )