Serialize/ deserialize with cattrs #40

Cattrs (https://pypi.org/project/cattrs/) is a powerful library for automatic serialisation/ deserialisation of hierarchical data. It seems to me that it could be valuable to use it to implement the various to_json methods (for example https://github.com/carbonplan/xarray-schema/blob/2c56d6ad2d4650ab06b09383a3cdd0bea0988ed7/xarray_schema/dataset.py#L70).