GeezerVison

This PR aims to solve #1156 by updating the current ss.lft function, adding a general lft function and adding more documentation.

Summary of main changes:

• Addition of new lft function with signature lft(sys1, sys2, [nu, ny], **kwargs) which can be used for any InputOutputSystem due to the use of interconnect if no statespace conversion is possible.
• Addition of preserving input and output signal names of the transformed system when using both the statespace and new lft function.
• Made docstring of ss.lft() self-contained. Here I added a new reference to a paper as the old one refers to a chapter (A.7) in the second edition of Skogestad which in mine does not discuss LFT's (chapter A.8 is about LFT's but does not contain an explanation of the star product). A reference to the star product does exist in the first edition but I thought this paper was more focused on the topic and also easier to find.
• Added narrative documentation for lft function.
• Added references to lft function in appropriate places.

AI Disclosure:
Claude was used to proofread and suggest corrections for documentation and verify the documentation and code compliance with the developer standards. Additionally, it was used to assess test coverage and draft some unittests after implementation of the test scaffolding. All suggestions were reviewed, understood and manually tested.