Create scripted version of lightcurve extraction from Notebook

Current code runs in Notebook.  Shift to use an organized set of functions in a package and clearly defined interfaces.

1. [x] Convert to basic Python script
2. [x] Refactor to functions
3. [x] Separate out calculation and plotting
4. [x] Write tests
5. [x] Ensure can be run from command-line or from other Python code with a analogous interface.