Replace license comments with SPDX ID in package.json

[localjo]

The standard way to include license information in a JavaScript project is by putting the SPDX license ID in package.json. I checked with @steadyflux and she confirmed that we can move the file-level license information to the project root, and NASA's Open Source License Agreement has a SPDX ID, so we should remove license-related comments from source files and put the following in package.json;

"license": "NASA-1.3"

License info in the README should stay there, since that's also a standard place to document a project's license.