Add implementation of the multi-fi cokriging MFCK (2 fidelity levels)

[mcastanoUQ]

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[relf]

Hi Mauricio, a few requests on your PR:

• you have to rebase your branch on top of current master (and resolve the small conflict),
• you have to pass ruff 0.6 (linting, formatting) on your code
• you have too many blank lines (please take a look at some other code in SMT)
• remove print statements (you can use SMT printer mechanism if needs be)
• you have to add some tests in dedicated tests/test_mfck.py file

[mcastanoUQ]

Thank you so much for the comments, I'll check every point