Additional columns that should be evaluated

[jsosulski]

For instance when comparing different covariance matrix estimation techniques I maybe want to know the determinant/condition/something of each method in order to maybe explain a difference in the outcome, i.e. score.

So far this PR only allows the user to define additional columns that should be evaluated. To actually fill these with values, a custom Evaluation implementation is needed.

[jsosulski]

This is a working implementation of adding additional columns to the results object. @sylvchev if we could merge this, I can push changes to #132 so that the information how large the data subset is, is no longer stored in the session string. Alternatively we can first look at this PR next week before going over #132.