[fmt] Expose function in Dumux::Fmt for variable format strings

In C++20 mode passing non-constexpr format string yields a compiler
error. In that case, we have to use vformat(varstr, std::make_format_args(a, b, c...))

dumux/io/format.hh

@@ -40,6 +40,9 @@ using Dumux::Detail::fmt::format;
 using Dumux::Detail::fmt::format_to;
 using Dumux::Detail::fmt::format_to_n;
 using Dumux::Detail::fmt::formatted_size;
+using Dumux::Detail::fmt::vformat;
+using Dumux::Detail::fmt::vformat_to;
+using Dumux::Detail::fmt::make_format_args;
 
 } // end namespace Dumux::Fmt