[DOC] Remove optional from docstrings

[TonyBagnall]

Describe the issue linked to the documentation

this is a minor change to allow new contributors to figure out the github workflow and make a first minor contribution. Our current standard doc comment for optional parameters is the

   window_length : int, default=10

however, in many places it is still listed as this

   window_length : int, optional (default=10)

it would be good to standardise across the toolkit. Feel free to change as few or as many as these as you want in order to make a contribution

Suggest a potential alternative/fix

No response

[griegner]

Happy to take on this issue, using vscode's find/replace to find regex pattern optional \(default=(.*)\) and replace with default=$1. I'll double check all replacements to verify that this procedure doesn't cause any problems with edge cases.