RFC: Formalize reader and writer API

[palemieux]

Formalize and normalize the reader and writer API:

def to_model(data_file: typing.BinaryIO, _config=None, progress_callback=lambda _: None) -> typing.Optional[model.ContentDocument]

def from_model(
  model_doc: model.ContentDocument,
  output: typing.BinaryIO,
  config: typing.Optional[imsc_config.IMSCWriterConfiguration] = None,
  progress_callback: typing.Callable[[numbers.Real], typing.NoReturn] = lambda _: None
  ):

Reading and writing from/to binary streams is required since some formats are text based and other are binary.

Closes #470