Expose `data` and `flags`
When having generated a saqc object from data, there is no straight forward way to checkout, for example, wich fields are available. Of course, i could check saqc._data.columns
or check the column names of the object i generated the saqc object with. But both options feel look odd to suggest in a tutorial. Intuitively, having generated a saqc object, the user should be able to obtain some basic informations on it, directly, via something like saqc.fields
or check the data out, via saqc.data[field]
, shouldnt he?