Paramtools

- Fixes bug where the original `defaults` dictionary was modified by a `pop` command. 53

- Make "validators" and "additional_parameters" optional. (52)

- Drop `schema.json`, and move it into a top-level `schema` member in `defaults.json`. (49)
- Rename `dims` to `labels`. (47)
- Rename `optional`—>`additional_members`. (49)
- Set a default value of zero for `number_dims` . (44)
- Handle simple parameter values. (45)
- Revamp documentation. (51)
- Documentation is now hosted at [paramtools.org](https://paramtools.org)