birdnotes.py: Objects to represent bird note files

In order to validate XML files against the schema, we need read access to the schema file in Relax NG form, that is, an XML .rng file, not in the .rnc compact form. To translate an .rnc schema into .rng form, use the freely available trang multi-format schema converter.

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# Manifest constants
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SCHEMA_RNG = "birdnotes.rng"