add option to pass in pre-computed signatures

[rchan26]

• Allow passing on pre-computed signatures into SignatureMahalanobisKNN rather than having to compute them
  • Signatures are only computed if X is provided and signatures is not in the .fit() method (similarly for conformance)
• Added docstrings for Add docstrings to all classes and methods #1 (change to numpy docstring format)
• Added type hints for Add type hints #2

[sz85512678]

Good work Ryan, thank you! Minor concerns about coding styles, let me know if you think what I said make sense... maybe not as it's late on Friday!

[rchan26]

Latest changes:

• Made the the error messages for not passing X or signatures or for not fitting the model yet global variables so you can update the error messages in one place (no duplication)
• Added back the documentation for sktime SignatureTransformer arguments and info for what we set as default arguments
• Moved the compute_auc methods in SignatureMahalanobisKNN to standalone functions outside the class in utils.py

[sz85512678]

Thanks Ryan for the changes :).