Expression files and instructions for building with Nix

I've been running pioneer on a NixOS system which is quite different to build on than a normal Linux system, so I thought I might include some support files and instructions.

I would have packaged it outright but because of the speed at which new releases come out, I decided to just make it easy to build from source.

On a NixOS system software is often build from source by the package manager, but the user does not have libraries or the autotools utilities available without explicitly requesting them. The file 'nix/env.nix' contains instructions for the package manager to install all of the dependencies for Pioneer and creates a script to load an environment where these dependencies are available.

This is related to, but does not depend on this issue https://github.com/NixOS/nixpkgs/pull/4352 (libassimp will be built from a local expression if it is not found in system the package manager).