New adaptive metamodel test functions could be added

[mbaudin47]

The four test functions are:

• f1(x): Franke’s function, d= 2
• f2(x): Hartman function, d= 3
• f3(x): Friedman function, d= 5
• f4(x): Gramacy & Lee function, d= 6

Reference : "Cross-validation based adaptive sampling for Gaussian process models"
Hossein Mohammadi, Peter Challenor, Daniel Williamson, Marc Goodfellow
https://arxiv.org/pdf/2005.01814.pdf

2005.01814.pdf