Simplify code using BOOST::variant.

[bangerth]

Following #13556, I looked up the Boost documentation at https://www.boost.org/doc/libs/1_78_0/doc/html/variant/reference.html#variant.concepts.static-visitor and realized that starting with C++14, one can omit writing a class on our own as a visitor, but that a generic lambda works as well. This patch does that, getting rid of a dozen lines or so.

/rebuild

[masterleinad]

/Users/runner/work/dealii/dealii/bundled/boost-1.70.0/include/boost/variant/detail/apply_visitor_unary.hpp:98:33: error: no matching function for call to object of type '(lambda at /Users/runner/work/dealii/dealii/source/base/table_handler.cc:119:7)'
        typedef decltype(true ? boost::declval< Visitor& >()( boost::declval< value_t >() )
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/runner/work/dealii/dealii/bundled/boost-1.70.0/include/boost/variant/detail/apply_visitor_unary.hpp:108:22: note: in instantiation of template class 'boost::detail::variant::result_multideduce1<(lambda at /Users/runner/work/dealii/dealii/source/base/table_handler.cc:119:7), boost::variant<int, unsigned int, unsigned long long, double, std::string>>::deduce_impl<boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<5>, int, boost::mpl::l_item<mpl_::long_<4>, unsigned int, boost::mpl::l_item<mpl_::long_<3>, unsigned long long, boost::mpl::l_item<mpl_::long_<2>, double, boost::mpl::l_item<mpl_::long_<1>, std::string, boost::mpl::l_end>>>>>>>' requested here
    typedef typename deduce_impl<begin_it>::type type;
                     ^
/Users/runner/work/dealii/dealii/bundled/boost-1.70.0/include/boost/variant/detail/apply_visitor_unary.hpp:114:22: note: in instantiation of template class 'boost::detail::variant::result_multideduce1<(lambda at /Users/runner/work/dealii/dealii/source/base/table_handler.cc:119:7), boost::variant<int, unsigned int, unsigned long long, double, std::string>>' requested here
    typedef typename result_multideduce1<Visitor, Variant>::type result_type;
                     ^
/Users/runner/work/dealii/dealii/bundled/boost-1.70.0/include/boost/variant/detail/apply_visitor_unary.hpp:135:98: note: in instantiation of template class 'boost::detail::variant::result_wrapper1<(lambda at /Users/runner/work/dealii/dealii/source/base/table_handler.cc:119:7), boost::variant<int, unsigned int, unsigned long long, double, std::string>>' requested here
    boost::detail::variant::result_wrapper1<Visitor, typename remove_reference<Visitable>::type> cpp14_vis(::boost::forward<Visitor>(visitor));
                                                                                                 ^
/Users/runner/work/dealii/dealii/source/base/table_handler.cc:118:12: note: in instantiation of function template specialization 'boost::apply_visitor<(lambda at /Users/runner/work/dealii/dealii/source/base/table_handler.cc:119:7), boost::variant<int, unsigned int, unsigned long long, double, std::string> &>' requested here
    boost::apply_visitor(
           ^
/Users/runner/work/dealii/dealii/source/base/table_handler.cc:119:7: note: candidate function [with arg:auto = int] not viable: expects an lvalue for 1st argument
      [](auto &arg) { arg = std::remove_reference_t<decltype(arg)>(); },
      ^
1 error generated.

[bangerth]

Ah, that's a shame. It compiles on my system, but let's not worry about it then. The code will disappear anyway once we use C++17.