Let grdtrack return nearest non-NaN value if unable to interpolate
|Target version:||Candidate for next minor release|
Sometimes we have grids with NaNs and wish to extract a value at given (x,y) points. If those points lands in a patch with NaNs then only NaNs can be returned. Alternatively, add option so that in such cases we search for the nearest node that is not NaN, and return its value and optionally its coordinates. Perhaps shomething like -Q[<radius>[unit]][+p] with optional search radius (default would considere any point in the grid) and modifier +p to replace input x,y with location of the nearest non-NaN node on output.