[FEATURE] Implement deprecated sktime interface on hcrystalball

[aiwalter]

Is your feature request related to a problem? Please describe.
The current sktime interface to hcrystalball is going to be deprecated soon. A solution to still use hcrystalball with sktime would be to move the wrapper from sktime package to hcrystalball package.

Describe the solution you'd like
Copy the existing code from https://github.com/alan-turing-institute/sktime/blob/main/sktime/forecasting/hcrystalball.py to hcrystalball. Tests from sktime are now also portable, whihc means they can be imported from the package and could be run on that class. See here: https://www.sktime.org/en/stable/developer_guide/testing_framework.html

[fkiraly]

The current sktime interface to hcrystalball is going to be deprecated soon.

@aiwalter, we have not decided on that yet!

See discussion here. sktime/sktime#2677

[MichalChromcak]

Thank you for mentioning it and opening the issue here. I hope to find the time reasonably soon to do the migration.