Add Accumulation to hydrology operations
fixed Accumulation.scala in hydrology

changed from using two stacks to using tuples.
Add combine() methods to DataSources.
Merge pull request #582 from joshmarcus/topic/ds_combine

Add combine() methods to DataSources.
Gave History a face lift.

- Also refactored the internal actors a bit.
Fixing server project, furthur refactors
More refactors to server actors.

- Rename Calculation to StepAggregator.
Fixed demo project
Added toJson to history
Fixes dev project

- Moved creating a router to the Server responsibility. Allows for the ServerActor to remain private to the package, which is good for future refactors of the underlying Actor system.
Merge pull request #584 from lossyrob/topic/history

Gave History a face lift.
Layer url work
Added ArgUrlRasterLayer, io.LoadRasterUrl
Merge pull request #585 from lossyrob/layer/url

Added RasterLayer to read args from URL
Added syntax for mapping over operations executed in parallel
Added more comments to GetHistogramArray.
Delete .SeqSourceBuilder.scala.swp
Took out unused parameter to FocalOp
Interim checkin of new focal operation infrastructure.
Resolve bugs and fix tests for new focal operation infrastructure.