background-only workspaces cannot be built as models

[kratsg]

Description

If we specify a background-only workspace, this passes the workspace.json schema, but we cannot build a Model from it, but we can build a Workspace from it.

Expected Behavior

We expect to be able to build a (background-only?) Model.

Actual Behavior

An error message like

pyhf.exceptions.InvalidModel: The parameter of interest 'mu_SIG' cannot be fit as it is not declared in the model specification.

Steps to Reproduce

Using the background-only JSON from the sbottom analysis causes this. Then running

import pyhf, json
w = pyhf.Workspace(json.load(file('BkgOnly.json')))
w.model()

Checklist

• Run git fetch to get the most up to date version of master
• Searched through existing Issues to confirm this is not a duplicate issue
• Filled out the Description, Expected Behavior, Actual Behavior, and Steps to Reproduce sections above or have edited/removed them in a way that fully describes the issue

[kratsg]

Idea: provide a background-only which (a background-only workspace still has to have a measurement object) which switches to using the lumi as the POI. The other idea is to fix the BkgOnly.json such that the POI is lumi.