add grid spacing and the ratio of grid spacing to chipsize0 so that adjacent chips can be dependent (finer sampling grid)

[leiyangleon]

• read GridSpacingX from the spacing of the DEM grid, while ChipSize0X is the smallest chip size. They are independent of each other, i.e. GridSpacingX can be either greater than or less than ChipSize0X
• define the ratio of ChipSize0X to GridSpacingX
• update NDC filter parameter FracValid to take account of the overlap between adjacent chips when GridSpacingX is less than ChipSize0X (note the overlap is constant when chip size iteratively progresses, thanks to the nested grid design)
• update NDC filter parameter FiltWidth to cover a consistent area no matter what GridSpacingX is used in relative to ChipSize0X
• update sparse search sample rate with the ratio of ChipSize0X to GridSpacingX to skip the same distance in the sparse search no matter what GridSpacingX is used in relative to ChipSize0X