Use generated data classes for parsing

[vladistan]

This is refactor of the original library.

• Use generated data classes produced by xsdata library to parse and validate XML file.
• Use top down parsing to construct dataclass based Atom + GreenButton forest
• Add some real unit tests
• Preprocess raw GreenButton data to make it easier to use (at the slight expense of performance)
• Use typed code eveyrwhere
• Handle edge cases not present in the sample files. Edge cases were found by running the type validator