Relicense from GPL-2 to AGPL-3

[dfalster]

This PR changes the license of odelia from GPL-2 to AGPL-3.

Reason

We are looking to vendor XAD, a C++ automatic differentiation library released under AGPL-3. GPL-2 and AGPL-3 are legally incompatible — they cannot be combined in the same binary — so incorporating XAD requires upgrading our license. AGPL-3 is the appropriate target because it is the license under which XAD is distributed and it is the most natural copyleft license for a combined work of this kind.

What changes

• LICENSE updated to AGPL-3
• DESCRIPTION updated from License: GPL-2 to License: AGPL-3
• CLA.md added, requiring contributors to grant the maintainers the right to sublicense future contributions

Practical impact on users

For typical use of odelia as a local scientific computing tool, the change from GPL-2 to AGPL-3 has no practical effect. The additional AGPL-3 obligation — that those running the software as a network service must make their source available — does not apply to ordinary research use.

Downstream

plant depends on odelia and will also need to be relicensed to AGPL-3 in a corresponding PR. All copyright holders on both packages have been consulted and have given consent. This to be confirmed via approval of this PR

[dfalster]

Hi @aornugent @richfitz ! As discussed, we're looking to relicense odelia from GPL-2 to AGPL-3. Can you give your formal approval for this change by approving this PR?

I've also added a Contributor License Agreement to set expectations for future license changes. for example, if we no longer need AGPL-3 and want to switch to something else.

[aornugent]

Happy with this change.