message: Documentation: make it easier to build only the documentation
  Introduce the possibility:
  ```
  Dbuild_documentation_only=ON
  ```
  so that only the doxygen documentation can be build and we don't have to resort to installing all kinds of sub packages like Boost or manipulate the CMake files:w
diff: +
  +option(build_documentation_only "Build only the documentation with doxygen." OFF)
  +
  cmake_minimum_required(VERSION 3.5)
  project(GUDHIdev)
  @@ -13,8 +16,10 @@ set(GUDHI_MISSING_MODULES "" CACHE INTERNAL "GUDHI_MISSING_MODULES")
  # This variable is used by Cython CMakeLists.txt and by GUDHI_third_party_libraries to know its path
  set(GUDHI_PYTHON_PATH "src/python")
  +if (NOT build_documentation_only)
  # For third parties libraries management - To be done last as CGAL updates CMAKE_MODULE_PATH
  include(GUDHI_third_party_libraries NO_POLICY_SCOPE)
  +endif()
  include(GUDHI_compilation_flags)
  @@ -52,7 +57,9 @@ foreach(GUDHI_MODULE ${GUDHI_MODULES})
  endforeach()
  endforeach()
  +if (NOT build_documentation_only)
  add_subdirectory(src/GudhUI)
  +endif()
  if (WITH_GUDHI_PYTHON)
  # specific for cython module
  