Switch from yarn to npm

[jniles]

When BHIMA started, we needed lockfiles to give predictive deployments, and yarn had them before npm. Now, npm has them and so much more! Additionally, yarn and npm have diverged in their handling of plug and play modules, and you need npm to install the latest yarn.

I don't see a need to yarn anymore. It adds additional complexity (one more thing to install and maintain) and provides little added value if any. I propose that we migrate from yarn to npm by:

1. Removing yarn.lock and committing package-lock.json
2. Updating our documentation that refers to yarn.
3. Updating our tests to only install npm and not yarn.
4. Updating our installation + release process to use npm instead of yarn.