Use openscm release

[znicholls]

Pull request

Update pymagicc to use the pre-release of openscm (mainly for testing purposes but the API should be pretty stable...). Also tidies up setup.py and Makefile as well as dropping join_timeseries (all of its functionality is superceded by OpenSCM).

Please confirm that this pull request has done the following:

• Tests added (N/A)
• Documentation added (where applicable) (N/A)
• 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>`_)

[rgieseke]

LGTM!