ASDF serialization

[ASDF - Advanced Scientific Data Format](https://asdf-standard.readthedocs.io/en/latest/) looks promising for serialization in Ocean (BQM/SampleSet in dimod's case).

It has the following features:

- A hierarchical, human-readable metadata format (implemented using YAML)
- Numerical arrays are stored as binary data blocks which can be memory mapped. Data blocks can optionally be compressed.
- The structure of the data can be automatically validated using schemas (implemented using JSON Schema)
- Native Python data types (numerical types, strings, dicts, lists) are serialized automatically
- ASDF can be extended to serialize custom data types

Libraries for [python](https://github.com/spacetelescope/asdf) and [c++](https://github.com/spacetelescope/asdf-cpp) are available.