Make Binder work again (at least for C++ 17 kernel)

The Binder as defined in May 2021 no longer worked (due to an issue from Xeus-cling - their demo Binder no longer works as well).

But modifying it with the compiler hint provided by Nicolas Thierry works: https://github.com/jupyter-xeus/xeus-cling/issues/413

I also moved the environment.yml file into a directory which makes its use more obvious, following suggestion from repo2docker documentation.

Solves #38 (closed)