Add notebooks installation extra

[znicholls]

Also has a few other tweaks:

• make black
• make test now gives a brief coverage report in the terminal
• updates the PR request template with our base url string

Pull request

Please confirm that this pull request has done the following:

• Tests added (N/A)
• Documentation added (where applicable)
• Example added (either to an existing notebook or as a new notebook, where applicable) (N/A)
• Description in CHANGELOG.rst added

Adding to CHANGELOG.rst

Please add a single line in the changelog notes similar to one of the following:

- (`#XX <http://link-to-pr.com>`_) Added feature which does something
- (`#XX <http://link-to-pr.com>`_) Fixed bug identified in (`#XX <http://link-to-issue.com>`_)

Closes #90

[znicholls]

@swillner @rgieseke I have no idea what is going on with Travis but the failure is for a PR which doesn't exist (93, see https://travis-ci.org/openclimatedata/openscm/builds/488755735). This PR, 92, seems to be fine but isn't appearing for some reason... https://travis-ci.org/openclimatedata/openscm/builds/488755170