Clarify interface (#33)

* Standardize cif example in parsnip module

* Rename table_labels to loop_labels

* Move parameterless read methods to properties

* Generalize getitem

* Clarify tip for getitem

* Lint parsnip.py

* Add pathlib to type hints

* Move loop_labels

* Move getitem to front

* Move pairs before loops

* Expand on docs

* Additional test for getitem

Author: janbridley