Use the GPL-3.0 license for all contracts

[neatonk]

I believe some of the contract code has been committed under the MIT license. Code in this repo currently using the MIT license should switch to GPL-3.0 with the approval of the code committer.

@josealonso and @dhruv035 do you approve of re-licensing the code you have committed to this repo under the GPL-3.0-only license?

If so, I will go ahead and change all solidity file headers to // SPDX-License-Identifier: GPL-3.0-only.

[josealonso]

I believe some of the contract code has been committed under the MIT license. Code in this repo currently using the MIT license should switch to GPL-3.0 with the approval of the code committer.

@josealonso and @dhruv035 do you approve of re-licensing the code you have committed to this repo under the GPL-3.0-only license?

If so, I will go ahead and change all solidity file headers to // SPDX-License-Identifier: GPL-3.0-only.

Yes, I approve it.