Vary relationship between requested raster CellSize and PDAL pipeline resolution

[echeipesh]

Currently the DEMRasterSource translated the requested cell size is translated directly to PDAL pipeline resolution:

geotrellis-pointcloud/pointcloud/src/main/scala/geotrellis/pointcloud/raster/ept/DEMRasterSource.scala

Lines 67 to 72 in ae576f8

	val expression = ReadEpt(
	filename = path.value,
	resolution = gridExtent.cellSize.resolution.some,
	bounds = s"([$exmin, $eymin], [$exmax, $eymax])".some,
	threads = threads
	) ~ FilterDelaunay()

We have observed that response latency is very sensitive to the query resolution: https://github.com/geotrellis/geotrellis-pointcloud/tree/master/benchmark

Because DEMRasterSource performs a Delaunay it can be expected to produce a sufficiently good result with sparser input and consequently in less time.

This issue is to take first steps:

• provide a fixed ratio between request and query resolution
• test and record the resulting performance and behavior

Note: This parameter should probably be parsed from the URI used to construct the DEMRasterSource so it can be easily threaded through.

Future work can incorporate more sophisticated logic for pipeline resolution selection based on the EPT metadata as mentioned in #62