Outlier detection: Support Cook's distance for SEM models

[rempsyc]

In the outliers paper, we currently have the following sentence:

 When no method is readily available to detect model-based outliers, such as for structural equation modeling (SEM), looking for multivariate outliers may be of relevance.

Mattan commented:

just found the influence.SEM::genCookDist() function the does just this. [...] I think we can add that to the [check_outliers()] function. [...] I think we need Brenton's input here (:

This issue is a reminder to add this method in the future, since we are removing the reference to SEM in the paper.