Executable: Add the DMP-Flagger

A couple of fixes/changes that allow the value dmp as a --flagger option:

• Plotting failed with flagger._flags of pd.DataFrame because of a comparison against a pd.Series
• The inital/fill-value of the flag fields quality_cause and quality_comment was changed to the empty string instead of NIL as that lead to the string "nan" in the final output (chain NIL -> np.nan -> "nan")
• Fix of a performance issue in initFlags with flag argument. In order to save a few (ugly) lines, I used a double pd.DataFrame.T - hack and it turned out, that transposing a pd.DataFrame is rather expensive (which makes sense, as every row needs to be casted to a common data type)
• Special casing the dmp flagger in the executable as I failed to fit both cases (single and multiple columns flags) into a common scheme within the limited time available.