Improve usability of raster rendering operations.

This pull request adds some helper operations to make it easier for a new GeoTrellis developer to render a raster.  It adds a new SimpleRenderPng operation that simply takes a color ramp and a number of class breaks, which it uses to generate quantile class breaks and the color ramps behind the scenes.  It also adds a set of default color ramps.

This pull request also adds comments and scaladoc to existing operations, and renames the render operation that expects a raster with RGBA integer values.