add CITATION.cff

[JoseFelipeFerrada]

Hi, thank you very much for your implementation of vector fitting!

It would be very useful if you could add a CITATION.cff file to accurately cite this module, besides citing Gustavsen's papers.

Below, i link to a useful resource regarding cff
Guide

[SebastianL18]

Thanks your recommendation. I added the .cff file to the repository, I hope it works well.

If you have any problem with this module or you have any suggestion, please let me know.

Best regards.