Pybiopax

What's Changed
* Add attributes to NucleicAcid reference BioPAX classes by bgyori in https://github.com/indralab/pybiopax/pull/48
* Added pathway_order and next_step to a list by tmushayahama in https://github.com/indralab/pybiopax/pull/49

**Full Changelog**: https://github.com/indralab/pybiopax/compare/0.1.3...0.1.4

What's Changed
* Add py.typed by bgyori in https://github.com/indralab/pybiopax/pull/45
* Add global tqdm config by bgyori in https://github.com/indralab/pybiopax/pull/46


**Full Changelog**: https://github.com/indralab/pybiopax/compare/0.1.2...0.1.3

What's Changed
* Add functions to get database references from a model by bgyori in https://github.com/indralab/pybiopax/pull/35
* Use empty list by default if list-type is None by bgyori in https://github.com/indralab/pybiopax/pull/37
* Enable passing a list of BioPaxObject to BioPaxModel by cthoyt in https://github.com/indralab/pybiopax/pull/38
* Improve BioPAX class attribute documentation by bgyori in https://github.com/indralab/pybiopax/pull/39
* Notebook demonstrating model construction by bgyori in https://github.com/indralab/pybiopax/pull/41

**Full Changelog**: https://github.com/indralab/pybiopax/compare/0.1.1...0.1.2

- Add a paper describing PyBioPAX in the paper folder
- Improve some tests to be version independent

- Improve the BioPAX object model by implementing Xreffable, Named and Observable as mixin classes.
- Implement BioPAX object reverse links (e.g., `participant_of`) as property attributes populated during deserialization.
- Implement a new `paths` module to find objects matching a given path specification as a string from a given starting object.
- Implement new API functions to query models from the Reactome, NetPath, EcoCyc, MetaCyc, BioCycm and HumanCyc web services.
- Implement API function to process owl.gz files.
- Extend testing to Python 3.6-3.9 and migrate to `pytest` for testing
- Add contribution guidelines
- Add a tutorial Jupyter notebook and two case study notebooks

- Fix participants attribute of Interactions to be list-like