Add Apache-2.0 license headers to source code

[JamieSlome]

As part of FINOS activation (https://github.com/finos/community/issues/361), all relevant source code files must reflect the project's license as a license header in each relevant file.

Tasks

• [ ] Identify all files that are missing the Apache-2.0 file header
• [ ] Append the Apache-2.0 license header to files

Reference

licenseheaders - a Python 3 tool to update, change or add license headers to all files