Fix to io.LoadTile for loading raster extent
Changed raster loading by name to accept data store name.

- If a layer is requested by only name (no data store name specified) and there is more than one data store that has a raster layer by that name an exception is thrown saying so.
Merge pull request #652 from lossyrob/loadtile-rasterextent

Fix to io.LoadTile for loading raster extent
Merged in upstream fix to LoadTile.
Merge pull request #653 from lossyrob/same-name-different-ds

Same name different ds