switch to MIT License

[jhamman]

This Pull Request proposes a license change to the VIC source code. In 2014, leading up to the major refactor that became VIC-5, we added the GPL-2 license to the VIC source code (#133). I vaguely remember discussing the choice of license with @bartnijssen but we didn't have public discussion about the choice.

Now, seven years later, I'm proposing to change VIC's license to the more permissive MIT license. While I'm not an expert on the legal specifics here, my understanding (and goal) is that this change will relax constraints on usage and third-party development, and perhaps more importantly, makes it easier for folks to use VIC in environments that don't support GPL licensed dependencies.

As a concrete example to motivate this change, I would like to incorporate part of the VIC source code into a separate project I'm working on. That project uses the MIT license and depending on VIC's implementation, would require adopting the GPL-2 license there as well (a non-starter).

Further reading

• https://choosealicense.com/licenses/mit/

• https://opensource.guide/legal/

• http://nipy.sourceforge.net/software/license/johns_bsd_pitch.html (discussion about a similar switch for Matplotlib)

• closes #xxx

• tests passed

• new tests added

• science test figures

• ran uncrustify prior to final commit

• ReleaseNotes entry

[bartnijssen]

I didn't check all the files (since the edit is the same for all of them).