added class IM7 to read davis PIV images
moved gradient and Q computation to separated functions (to be used without having to do a full generateFields() call
added copy function to PointProbe
added modifier function to PlotField (e.g for scaling), and argument passing to PlotVelocityVectors
small changes to SurfaceTimeSeries
added Okubo-Weiss Q computation to Surface
lin. interpolation with lin. extrapolation added to Math