Local operations act transparently on tiled rasters.

This pull request makes local operations work with all rasters, i.e. individual and in memory or tiled and distributed.

As part of the effort to make local operations work transparently on tiled rasters, the local operations are redefined in this pull request to use new helper operations in place of directly invoking methods on actual raster instances. This keeps the abstraction at a single level and gives us a single point in which we can handle operations on distributed transparently rasters. 

As this involved updating every local operation, I ended up fixing some naming issues and bugs as well.

This work needs further testing and benchmarking.