Update `BATSModel`, `TBATSModel` to handle integer timestamp

[d-a-bunin]

🚀 Feature Request

In #178 it was found that tbats models fail on data with integer timestamp. It could be fixed.

Proposal

Fix tests tests/test_models/test_inference/test_forecast/TestForecastOutSample for models:

• BATSModel
• TBATSModel

Error:

ValueError: Model is not fitted! Fit the model before calling predict method!

The problem is with _TBATSAdapter.forecast which checks if self._freq is None which is now a valid value.

• The default value for self._freq should be changed.
• The check on fitted model should be changed.

Test cases

• Move fixed tests in tests/test_models/test_inference/test_forecast/TestForecastOutSample from fail to success category
• Move fixed tests tests/test_models/test_inference/test_predict/TestPredictInSampleSuffix from fail to success category

Additional context

Blocked by #178.

[d-a-bunin]

Closed by #195.