Paramtools

- Update custom fields for Marshmallow 3.0.0rc7 API changes. (66)

- Bug fixes related to extend capability and its interactions with the `Parameters` instance's state. (63)

- Apply extend capability to user adjustments. (60)
- Add flexibility to the `Parameters.extend` by extending at the value object level instead of the `label_to_extend` value level. (60)
- Refactor `Parameters._select` method into the `select` module. This refactor allows the user to implement custom comparison functions similar to what can be done with `pandas.DataFrame.loc`. (60)

- Add array extension capability. This is helpful for filling out a parameter's values without having to specify redundant data in `defaults.json`. Check out the [extend docs](https://paramtools.org/api/extend/) to learn more about this feature. (#58)
- Add [API guide](https://paramtools.org/api/guide/). (#58, 59)

- Preserve order of parameters. (7a4f949d4adabe0eab47e4627452bcf01b07bdcc)

- Add serializable flag to specification method. (54)
- Rename schema 'builder' to schema 'factory.' (55)