Waterlevel differences & bed height differences: Add new model LinearInterpolated intented to unify the two very similiar calculations. The focus of the current implementation is correctness and not speed! The fact that the data sets more mostly sorted by station is not exploited. Doing so would improve performance significantly.

SYMBOLSET
SYMBOL
  NAME 'point'
  TYPE ELLIPSE
  POINTS
    1 1
  END
  FILLED TRUE
END
SYMBOL
  NAME "square"
  TYPE VECTOR
  POINTS
    0 0
    0 1
    1 1
    1 0
    0 0
  END
  FILLED TRUE
END
END