CMake cleanup in SearchInstalled

cmake/bornagain/modules/SearchInstalledSoftware.cmake

 # Search for installed software required by BornAgain
 
-# === obligatory packages ===
-
 find_package(Threads REQUIRED)
-
-# --- math packages ---
 find_package(FFTW REQUIRED)
-find_package(GSL REQUIRED) # revert this when issue 1404 is resolved
-find_package(YamlCpp05 REQUIRED)
+find_package(GSL REQUIRED)
 
 # --- Eigen3 is a git submodule; use system eigen if this submodule was not properly cloned ---
-
 set(EIGEN3_INCLUDE_DIR "${CMAKE_SOURCE_DIR}/ThirdParty/Core/eigen3" CACHE INTERNAL "")
 if(NOT EXISTS "${EIGEN3_INCLUDE_DIR}/signature_of_eigen3_matrix_library")
     unset(EIGEN3_INCLUDE_DIR CACHE)
@@ -72,7 +66,6 @@ if(BORNAGAIN_PYTHON OR BORNAGAIN_GUI)
     endif()
 
     find_package(Numpy REQUIRED)
-    find_package(PyYaml REQUIRED)
 endif()
 
 # --- Swig ---