Make MultipleDatabaseImageStore a direct part of the inheritance hierarchy
Make 'get_tile' an abstract member of ImageStore

Rename some variables in PtiffImageStore.get_tile to be more readable.
Make ptiff's "getcoords" return a tuple instead of a list

When Python functions return multiple values, it is implicitly a tuple.
A list is mutable and should not be returned unless specifically necessary.
Remove deprecated references to "Database" and "PtiffTileStore"

Generally, references to "Database" were generalized to "ImageStore", while
references to "PtiffTileStore" were kept specific and changed to
"PtiffImageStore".
Merge branch 'image_store'

* image_store:
  Remove deprecated references to "Database" and "PtiffTileStore"
  Make ptiff's "getcoords" return a tuple instead of a list
  Make 'get_tile' an abstract member of ImageStore
  Make MultipleDatabaseImageStore a direct part of the inheritance hierarchy