Dakota issue 18: handle anisotropic GP modeling

[JavierGOrdonnez]

Parent epic: #12
Source: ITISFoundation/mmux_python#3

Current status: Not addressed. No anisotropy diagnostics or input-space transformation strategy is exposed.

Acceptance criteria:

• Define supported anisotropy detection or user-specified transformations.
• Make transformations inspectable and invertible at the public API boundary.
• Add a synthetic anisotropic benchmark and document limitations of global GP models.