Use Nix for package and configuration management

[smiths]

In the first iterations Drasil will expect the user to provide the correct external libraries for use with the generated code. For instance, if an ode solver library is needed, the user will need to install the correct version in the correct location. In the future, Nix could be used to manage the packages and configurations.

[cd155]

These two smart constructors are used to specify the package names and package versions. https://github.com/JacquesCarette/Drasil/blob/adfc15f13c7cf81101c1dbab97a6aa61f10a4ec6/code/drasil-code/lib/Language/Drasil/Data/ODELibPckg.hs#L37-L49