Related Issue or bug

Info about Issue or bug

• Enhances the implementation requested in [ENH]: Improve seasonality type detection in time series #3232
• Relaxes some requirements for and related to sktime

Describe the changes you've made

• Seasonal type detection is now based on FPP https://otexts.com/fpp2/seasonal-strength.html
  • Additive Strength is based on the formulae in the book
  • Multiplicative Strength is a variant of the same formulae.
• TODO: Need to see how good this method is. It does not always give the best result, but may still be better than what we have right now.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, local variables)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Unit test has been added

Describe if there is any unusual behaviour of your code(Write NA if there isn't)

A clear and concise description of it.

Checklist:

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.