RNAmf - Recursive non-additive emulator for multi-fidelity data

Performs RNA emulation and active learning proposed by Heo and Sung (2023+) <arXiv:2309.11772> for multi-fidelity computer experiments. The RNA emulator is particularly useful when the simulations with different fidelity level are nonlinearly correlated. The hyperparameters in the model are estimated by maximum likelihood estimation.