Code Clean up.

Document if operations do not support doubles.
Fixed bugs:
  - AsArray was not working with Ints. Also added AsArrayDouble.
  - SurfacePointCalculations (Hillshade,Aspect,Slope) were not checking
    for NoData correctly.
  - Double versions of Zonal operations (Sum,Min,Max) were not dealing
    with doubles properly.
Random documentation and formatting.

Closes #336
Merge pull request #339 from lossyrob/wip/double-doc

Code Clean up.