Preparing "stand alone" variant

[IanGrimstead]

Using new localhost branch as an interim "develop" variant, as this is a large feature change that will impact a lot of the codebase.

Aim is to enable GCP and "local host" variants in a single codebase, with common code in a shared library, GCP & localhost in isolated modules.

Broken down into tasks:

1. Refactor code to share common base, split into GCP & localhost #12
2. Create download files & update sources for localhost #12
3. Create process_images for localhost, investigating database for MS Windows via Python #14
4. Port scripts to localhost if needed (covered with #14)
5. Recover unit test coverage after spike (covered with #14)
6. Ensure GCP still functions cleanly (covered with #14)