Update the PR checklist

[mo-nabilbrice]

Similar to checking the documentation renders correctly, it could be useful to include an explicit checklist item in the PR checklist to ensure the rose GUI renders as expected (including without errors). This checklist item will become more important with further development on the CMEW configuration options, e.g. issue #189.

While we're at it, update the unittests checklist item to ensure it gets checked just before approval.

Also, update the current items that present a choice to include two independent choices, so that a single choice can be selected.

[alistairsellar]

I suggest expanding this checklist item to something that parallels the user documentation item. E.g. "Has Rose metadata related to the change been updated appropriately, and does the Rose GUI render as expected?"