Make boost optional for all user. When BUILD_USE_BOOST is enabled cmake will...

Make boost optional for all user. When BUILD_USE_BOOST is enabled cmake will look for the boost header only libraries. Afterwards boost needs to be linked to the required library.
e.g:
if(BUILD_USE_BOOST)
    target_link_libraries(${library_name} PRIVATE Boost::boost)
endif()

CMakeLists.txt

@@ -46,7 +46,7 @@ option(BUILD_VF_CPU "Build VirtualFluids cpu variant" OFF)
 option(BUILD_VF_GPU "Build VirtualFluids gpu variant" OFF)
 
 option(BUILD_USE_OPENMP "Build VirtualFluids with openmp" ON)
-
+option(BUILD_USE_BOOST "Build VirtualFluids with boost" OFF)
 
 # vf gpu
 option(BUILD_VF_GPU          "Build VirtualFluids GPU"     ON )
@@ -166,6 +166,19 @@ endif()
 
 find_package(MPI REQUIRED)
 
+# boost
+IF(BUILD_USE_BOOST)
+    list(APPEND VF_COMPILER_DEFINITION BUILD_USE_BOOST)
+
+    set(Boost_USE_STATIC_LIBS ON)
+    set(Boost_USE_MULTITHREADED ON)
+    set(Boost_USE_STATIC_RUNTIME ON)
+
+    # minimum boost version: 1.60
+    # no packages specfied - only headeronly libraries
+    find_package(Boost 1.60 REQUIRED)
+ENDIF()
+
 add_subdirectory(src/logger)
 add_subdirectory(src/basics)
 #add_subdirectory(src/mpi)