Documentation for parameter-related properties of `_ModelConfig`

[alexander-held]

Summary

There are a few different things available in a model.config for accessing parameters:

• model.config.parameters for an alphabetically sorted, non-expanded (staterror/shaperror) list,
• model.config.par_order for a non-expanded list in the order that pyhf uses internally when handling and returning parameters,
• model.config.par_names() for an expanded version of the list in the order used internally.

Only par_names() shows up in the _ModelConfig documentation at the moment. parameters presumably comes from the _ChannelSummaryMixin mixin, but I do not see this class in the documentation (if that was there, clicking through to it from the _ModelConfig page could be a convenient way to find additional properties coming from the base class).

model.config.parameters may not be that commonly needed, and could be obtained from model.config.par_order. The latter is very useful when using model.config.param_set().

See also #1024 for a few more points related to parameters vs par_order.

Documentation Page Link

https://pyhf.readthedocs.io/en/v0.6.3/_generated/pyhf.pdf._ModelConfig.html

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[matthewfeickert]

Thanks @alexander-held — 100% agree with you here. :+1: