spdx: change agpl to sspl

[sambacha]

SPDX

this pull request changes // SPDX-License-Identifier: MIT to reflect the newer license of /// SPDX-License-Identifier: SSPL-1.0

note - /// usage for solc warnings even tho

AGPL vs SSPL

See a diff here: AGPL_vs_SSPL

tl;dr SSPL addresses issues of SaaS usage, mainly distributing the software over a network and not disclosing interfaces, etc

Previous PR

#47
closed previous PR, #47 to address styling changes

Notes

in the future, there should be a 'dummy' git account in which automated processes (such as CI/CD/etc) can be assigned to, e.g. git config user.email [email protected] in order to assist in the review process / diff'ing / etc, esp. considering increasing amounts of outside contributors, and other automated services (ci pipelines, etc)

Optional Header

Copyright 2020 - yearn protocol contributors  

This program is free software: you can redistribute it
and/or modify it under the terms of the Server Side Public
License, version 1, as published by MongoDB, Inc.
See the Server Side Public License for more details. You should
have received a copy of the Server Side Public License
along with this program.

<http://www.mongodb.com/licensing/server-side-public-license>.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty
of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 

[sambacha]

closing in favor of #61