assimp: initial package expression for version 3.1.1

Open Asset Import Library is a library for importing 3D models in various formats

It might seem odd to make a PR for just a library, but I'm packaging it for the sake of the Pioneer space sim http://pioneerspacesim.net/. The game is released in form that is intended to be run from a git repo, so I'd like this dependency in nixpkgs and then for now run the game with an env expression that upstream can include in the game's repo.