fix: Update all APIs to pyhf v0.7.0

[matthewfeickert]

Resolves #33

Amends PR #29

* Update all APIs to be consistent with v0.7.0.
   - Remove comment about making measurement on the fly with
     `workspace.get_measurement` as no longer relevant.
   - Remove all use of `workspace.parameters`, now
     `workspace.model().config.parameters`, as the notebooks
      are focusing on workspace attributes, not model config
      attributes.
* Amends PR #29

[kratsg]

I'd just drop the ws.parameters calls -- since the relevant portion of the tutorial is only talking about workspaces, not models.

[matthewfeickert]

I'd just drop the ws.parameters calls -- since the relevant portion of the tutorial is only talking about workspaces, not models.

@kratsg done.

[matthewfeickert]

Merging as have approval from Giordon over Skype.