Use MAX resampling method for PNG pyramid

[echeipesh]

geotrellis-road-distance-sdg/src/main/scala/geotrellis/sdg/OutputPyramid.scala

Line 43 in 66baeab

val pyramid = Pyramid.fromLayerRDD(baseLayer, Some(baseZoom), Some(0))

This will preserve the biggest change to highest zoom level.
The default is NearestNeighbor which will basically randomly pick out of the neighbors, sometimes picking the lower value pixel, process repeats and at max zoom you're very likely to have the min value, washing out any difference.