Use a trivially copyable struct for TBTables::Entry instead of a tuple.

[Sopel97]

Fixes #2673

[vondele]

Thanks!

[gvreuls]

Joachim Müller (JojoM) reports on fishcooking that his workers get a compiler error on the use of std::is_trivially_copyable. Perhaps get rid of the static assert on line 417?

[vondele]

Okay.... GCC versions < 5 do not support std::is_trivially_copyable from the C++11 standard.
and he's using 4.9, so we might remove it as a workaround.