Numerapi

- added `get_account_leaderboard` to fetch account-level leaderboard
- added 'models_of_account' to fetch all models of an account
- 'SignalsAPI.ticker_universe': stop using hardcoded URL
- remove `set_stake_type` - no longer relevant
- deprecate 'SignalsAPI.download_validation_data'
- code style fixes
- fix docs

- signals: add 'list_datasets' and 'download_dataset'

- add `bmcRep` and `mmcRep` to `get_leaderboard`

- fix `get_leaderboard`

- added 'corr20V2Rep' to 'get_leaderboard'
- deprecate methods related to v2 and v3 datasets

- added `download_submission` to download a CSV of a previous submission
- added `submission_ids` to fetch all submission_ids of a model
- minor fixes