RGB styling configuration

[pomadchin]

After implementing MAML RGB Composite operation and all the nessesary color correction nodes (at least rescaling) we'd have to expose this functionality into the GeoTrellis server.

The parameters used for the default scaling should be possible to set through the application.conf file (potentially through the MAML expression). It should be also possible to override set parameters through the extended GetMap / GetCoverage get arguments.