station_range 超过 10，外部的栅格就没有数据填充

[xiaobai1017]

let pm25IDWOptions = {
// opacity - the opacity of the IDW layer
// cellSize - height and width of each cell, 25 by default
// exp - exponent used for weighting, 1 by default
// max - maximum point values, 1.0 by default
// gradient - color gradient config, e.g. {0.4: 'blue', 0.65: 'lime', 1: 'red'}
opacity: 0.5,
maxZoom: mapOptions.maxZoom,
minZoom: mapOptions.minZoom,
cellSize: 5,
exp: 2,
gradient: pm25Gradient,
dataType: 2,
station_range: 20,
minVal: 0.0,
maxVal: 150.0
};

[HuangLiPang]

@xiaobai1017
I think this is because the drawing algorithm and the indicator algorithm are not the same. Feel free to change it and make a PR!