Extract new outputs from previous save files

[eckelsjd]

Expected Behavior

• It should be possible to build a surrogate over new outputs if they are extractable from previous surrogate save files, even if they weren't originally defined in surr.coupling_vars.

Actual Behavior

• If you want the surrogate to predict a certain output, it must be defined with SystemSurrogate.coupling_vars up front, before training.
• This means you cannot predict a new output without retraining a new SystemSurrogate, even if that output is already computed and saved to file.