More detail needed in emulator_from_data help doc

[johnpaulgosling]

The help documentation for emulator_from_data states "The hyperparameters of the correlation structure [are] determined using a combination of the Durham heuristic and maximum likelihood estimation." The "Durham heuristic" is not well known. It would be beneficial to have a reference or a brief explanation.

[andy-iskauskas]

Agreed - rather than go into detail about the Durham heuristic, I've instead combined vagueness and descriptiveness and said that we use maximum likelihood with a restriction to the correlation length to avoid it going to 0 or (effectively) infinity.