Make more functions source-target ready

I adapted the follwing handles_target=True functions:

• funcs.outliers.flagMVScores
• funcs.outliers.flagCrossStatistic (which should actually be named flagCrossStatistics, right?)
• funcs.drift.flagDriftFromReference

These functions are not covered by the general source-target workflow, so I implemented the following behavior:

• assert len(field) == len(target) as all of these functions seem to modify all given fields
• drop existing targets
• copy all fields to there respective targets with copyField (i.e. we preserve histories)

I hope I didn't mess up, the general functionality, but I guess a critical review would be good @luenensc .