check the covariateSettings object

[jreps]

It seems that users may be able to specify an incorrectly formatted covariateSettings and the code runs but then hangs when FeatureExtraction is called. It is difficult for users to figure out the cause of the issue.

We should add a check in the package for the covariateSettings to make sure it is a covariateSetting object or a list of covariateSettings. Then we can print out the issue if the covariateSettings is in the wrong format.