Move models into package namespace

[ethanplunkett]

Move models environment into package namespace and don't load them during package load. This is simpler than what we are doing now.

• In R/models.R : models <- new.env()
• In load_models()
  • Drop code that creates the environment.
  • Update conditional for loading models to only check if the models are in the environment.
• Delete .on_load()
• Add text to readme to explain that models should be explicitly loaded by calling load_models() while configuring the API.