Add walberla variant

Can we have std::variant/boost::variant in waLBerla? We can use boost::variant as a fallback until we move to C++ 17.

I was using std::variant for my discretization/boost::units module whenever I can either take a physical pressure in pascals or a physical density in kg/m^3 (will commit this maybe next week). std::visit could also be useful to write terse code that dynamically selects the right template at run time.

I didn't add a try_compile yet to CMakeLists.txt to decide whether to take std::variant or boost::variant.