Numerapi

- make `get_current_round` available in SignalsAPI
- add `download_validation_data` to SignalsAPI, to download the latest validation
data, historical targets and ticker universe

- deprecate multi-tournament handling
- `get_models` now returns the list of models depending on the tournament you
are working on - numerai classic (NumerAPI) vs numerai signals (SignalsAPI).
This is necessary after the recent "model split".

- `daily_user_performances` add `fnc`
- `daily_submissions_performances` filter all-None items

- improve docstrings and signals example code
- remove deprecated `get_v1_leaderboard`
- remove deprecated `get_stakes` & `get_submission_ids`
- Fix default file path for `download_latest_data` (37)
- test suite: fixes

- fix `stake_change` call by adding `tournament` parameter (32)
- add `tournament` parameter to all stake related endpoints
- code style checks with `flake8`
- Remove header from signals universe (33)
- Add `get_latest_data_path` and `download_latest_data` (35)

- Add additional metrics to `submission_status` (30)
- signals: add `mmc`, `mmcRank` and `nmrStaked` to `get_leaderboard`
- signals: add `totalStake` to `public_user_profile`
- signals: add `stake_get`