Ntropy-sdk

Added
- Made `account_holder_id` no longer required (but still usable). Transactions can now be enriched by only providing `account_holder_type`, and will not be tied to the ledger of any `account_holder`. Transactions can also be sent without `account_holder_id` and without `account_holder_type` but will not be labeled.
- Added `transaction_type` attribute to the `EnrichedTransaction` object which contains the expected type of expense for the enriched transaction (`business`, `consumer` or `unknown`)
- Add models package to integrate with the new custom fine-tuning service. Includes `CustomTransactionClassifier` as a trainable model, fully compatible with scikit-learn Estimator and Classifier interface.
- Allow initialization of the SDK using the environment variable `NTROPY_API_KEY`
- Removed `benchmark.py` file and corresponding documentation

Added
- `create_account_holders` parameter for `SDK.add_transactions`. When set to `True`, if a `Transaction` contains both an `account_holder_id` and `an account_holder_type`, that account holder will be created during the `add_transactions` method.
- `SDK.get_account_holder` method to retrieve, if it exists, stored information about one account holder.
- `confidence` attribute to the `EnrichedTransaction` object.