docs: readme: Explain model-construction in Quickstart and improve
doctests in it.
core: Experiment: hide self._model as private.
tests: wltp_db: Rework comparisons to reuse iteration code (prepare for
caching)

* model: Improve docs for future comprehensive model-build procedure.
* docs: Add more TODOs in CHANGES.
* docs: Remove all :create: fields from file-headers.