temporarily reintegrate libformfactor in BornAgain

To facilitate forthcoming API changes.

Once the API is stable, we need to

• overwrite libformfactor/ff with BornAgain/ff
• remove BornAgain/ff
• remove add_subdirectory(ff) from BornAgain/CMakeLists.txt
• uncomment find_package(formfactor REQUIRED) in BornAgain/cmake/BornAgain/Dependences.cmake

Base/CMakeLists.txt

@@ -22,7 +22,6 @@ target_link_libraries(${lib}
     PUBLIC
     ${GSL_LIBRARIES}
     ${FFTW3_LIBRARIES}
-    formfactor0
     )
 target_include_directories(${lib}
     PUBLIC