[ENH] introduce revised version of ETS (#2834)

* first version for correctness testing

* timing

* examples

* new tests

* refactor predict

* final ETS

* arima first

* docstrings

* docstrings

* Convert error, trend and seasonality types to strings

* Automatic `pre-commit` fixes

* Add Type hints back in

* Fix Type hints

* Fix Linting issue

* Automatic `pre-commit` fixes

* Update test_ets to use strings for error, trend and seasonality types

* Automatic `pre-commit` fixes

* switch to fastmath

* remove _arima, incorrectly introduced here

* all int or str parameters

* refactor variable name

---------

Co-authored-by: alexbanwell1 <31886108+alexbanwell1@users.noreply.github.com>

Author: TonyBagnall