Ensure that remote operations are not only lazily evaluated.

This checkin adds a Force() operation that will force the evaluation
of any operation, although at the moment that only applies to
Raster operations.
Merge pull request #495 from joshmarcus/topic/lazy

Ensure that remote operations are not only lazily evaluated.
Merge remote-tracking branch 'upstream/master' into cache/fix
Set in RasterLayerInfo whether or not the layer is cached.