Complete documentation of new package

[axch]

We should document the following topics

• [ ] Usage and options for the command line executable, once that exists (#102).
• [x] Format of the configuration file, including named input generators
• [x] User API for custom input generators
• [ ] Developer API for how to add more named input generators
• [ ] Workflow for reading the generated output (core.interpret_raw_file, non-portability thereof across systems, etc).
• [ ] Describe each parameter, including units (#58), in
  • The online help of the executable (once that exists)
  • The generated documentation of the top level Python function
  • Comments in the example aronnax.conf file
• [ ] Ideally, arrange the mechanism for that description to minimize duplication of words across these presentation channels (so they stay in sync).