Model env presets

Inside of syberia models, the local variables `model_version`, `model_name`,
  and `output` are now available. The former two are the full model version
  (e.g., "default/en-US/1.0") and model name ("1.0"), respectively, whereas
  `output` is a helper function that outputs to the `tmp` directory relative
  to the root of the syberia project. For example, `output("blah.csv")` 
  yields "<syberia_project_root>/tmp/default/en-US/1.0/blah.csv". This can
  be very helpful for temporary output files, like a copy of the model or
  presentation files (like plots). The behavior of output may change in the
  future, but for now it is fixed to the "tmp" directory.