Refactor model code

[j08lue]

• [ ] Bring into systematic package structure
• [ ] Isolate actual ET model code from interface
• [ ] Make interfaces for model all-numpy (related to above)
• [ ] Add some rudimentary tests

[j08lue]

This PR was if we were to refactor the Python code. Now that the aim is to translate it all to GEE, this is obsolete.