Add RMSE and MAE sdmTMB_cv examples to cross validation vignette

[seanhardison1]

Hello! I'm a big fan of the sdmTMB_cv function, but I find the likelihood-based measures that are currently returned with the function to be a bit unintuitive. For a future iteration, it would be nice to have the function also return RMSE and MAE (by fold? averaged across folds?) in addition to the likelihood-based measures, or perhaps allow for user defined functions.