tracking changes during preparation

[nmprista]

We need a way to track changes in the data. One possibility is to add a new colum that track changes on the RDBESdataobject. Another is to add a columns that track changes introduced by generateMissingSSRows, generateZerosUsingSL, generateNAsUsingSL.

[rix133]

I think that code run on an data object is a track of changes unless you need to treat rows that have changes differently in estimation. This case seems a slippery slope to me as the function would behave differently on the same data that is reached in different ways.