Test predictions

[femiguez]

Perform additional tests for predictions.

• With newdata at different hierarchical levles. Note that some of these do not make sense, but I need better messages
• With spatial covariance structures (at different levels), gls, gnls, nlme, lme
• Expand section on 'Prediction' in the 'Bootstrapping' vignette

[femiguez]

Need to write an article about this, maybe separate from the vignette