(issue1755) Interpolate BedQuality Results This adds an interpolation function to each various bedQuality result class. Imho this is ok as the interpolation function can be seen as part of the result. The interpolation function is initalized on first use and can be accessed through get.*Interpol functions.

select.abflusstafel = \
    SELECT ABFLUSSTAFEL_NR, \
           ABFLUSSTAFEL_BEZ, \
           GUELTIG_VON, \
           GUELTIG_BIS, \
           PEGELNULLPUNKT, \
           AT.BFG_ID \
    FROM ABFLUSSTAFEL AT \
    WHERE MESSSTELLE_NR LIKE :number