Add Apache license

[j-wags]

Some users have asked whether our software can be used under different licenses. In particular, some people are most comfortable working with Apache-licensed software. I don't have any problem with this, and other efforts seem to have a way to present multiple licenses and say "here they are: pick one". So, this issue is to:

• Figure out the wording/legal mechanism to declare a "pick one" license system
• Figure out what sort of consent we need to get from this repo's contributors
• Actually gain those consents
• Open a PR to add the new license and the "pick one" mechanism

[j-wags]

Based on a lawyer's unofficial comments, multi-licensing is very complicated and can have far-reaching consequences. I'll wait for another request to prioritize this.

[j-wags]

Update: Based on a lawyer's somewhat official, written comments, multi-licensing is very complicated and can have far-reaching consequences. We should not naively dual-license this codebase. I think two ways to move forward on this are:

• If we do want to provide this code under the Apache license, consider a structure like "sub-releases", where during each release, we make a standard (MIT) release, and then modify that, and produce a descendant Apache release. I don't fully understand the details of this, but it was implied other orgs seem to use a mechanism like this to safely dual-license.
• Currently a license change would require tracking down and gaining agreement from all historical contributors. If OpenFF can become its own legal entity, it can own things. So, if OpenFF ever becomes a standalone OSS organization, we should ask the contributors to this codebase (myself included) to give ownership of their contributions to that organization. We would also want to begin including cpython-style contributor agreements in PRs from new users to ensure they understand that the OSS organization owns the code.

[mattwthompson]

There hasn't been movement on this in three years so I have to say it's not likely to move anytime soon. If we truly want this we can hire lawyers ourselves and act on it