Update install setup

[znicholls]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Description in RELEASE_NOTES.md Added

Adding to RELEASE_NOTES.md (remove section after adding to RELEASE_NOTES.md)

Please add a single line in the release notes similar to the following:

- (#XX)[http://link-to-pr.com] Added feature which does something

Description of PR

A way to update the install setup to make everything easier and more automatic for the user.

For example, this now works

git clone git@github.com:IAMconsortium/pyam.git
pip install -e .[test]
pytest

[gidden]

can you add the license per #121 please?

[znicholls]

to get the release notes right I should probably rebase onto #128 (or master once #128 is merged)