Add basic LJ potential*

- Add PairPotential Abstract class
- Add Lennard-Jones potential that should work with both CUDA and C++
  code
- Add tests on HOST side for LJ potential

tests/unit_tests/CMakeLists.txt

@@ -5,5 +5,5 @@ add_executable(${NAME}_tests
 )
 
 target_link_libraries(${NAME}_tests gtest gtest_main)
-target_link_libraries(${NAME}_tests ${CMAKE_PROJECT_NAME}_lib)
+target_link_libraries(${NAME}_tests ${CMAKE_PROJECT_NAME}_cuda_lib)
 add_test(NAME ${NAME}Tests COMMAND ${CMAKE_BINARY_DIR}/tests/unit_tests/${NAME}_tests)