- env variables are defined in 'config/env.py', this includes:
  - their default value
  - their documentation

- the loaded variables are stored as attributes of this module
  (eg: config.env.ALLGO_DEBUG)

- all variables are guaranteed to be set once the module is loaded
  thus, the config.settings just has to use env.ALLGO_XXXXX

- the sphinx documentation of the variables is generated directly
  from the source code

- the plumbing is done by the config.env_loader module (while loads
  the variables, and patches the docstring of the config.env module)