core: add fields to flagger where they are created and not within the core

We currently add flagger fields within the core, which leads to the following problem:

Let's say we want to add a new field to the flagger with flagGeneric("x", func y: y < 100). With the current mechanism an empty column is added to the flagger-DataFrame and as this column is empty won't receive any flags ever. So even after the above test yielded positive results, the column flagger.getFlags("x") is always an empty Series.

In order to fix this, I moved the adding of flagger fields from saqc.core._saqcCallFunc to the two places where we currently have the possibility to add new columns, namely flagGeneric and procGeneric