Release v0.0.7

- Add documentation for design considerations

- Update dependencies
- Update time library from chrono to time
- Update dee error handling on HTTP remotes
- Update tlock_age to v0.0.2 for improved performance

- Fix Chain error handling on wrong URL string

- Add coin_clip example for drand_core
- Add `get_by_unix_time` method on drand HttpClient

- Update dependencies
- Update drand_core public API to expose HttpClient
- Update bls12-381 library to arkworks/curves

- Fix Chain error handling on wrong URL string

- Remove Chain struct abstraction