fixes the issue #490 such that SparseDataset works with the new style of the iterator with data_specs
adding a test unit for issue #490 in addition to completing the SparseDataset class
removing a comment
correcting comment #490
updating issue #490
updating the issue #490 to add module_level docstring
updating ticket #490 to cover the comments of Xavier.
updating issue #490 to apply comments of Xavier
applying modification for ticket #490 to use Train object.
changing a comment for issues #490
adding comments to the dummy classes in test_sparse_dataset (ticket #490)
Updating the test_sparse_dataset for ticket #490 to update module_level and
class level comments
updating the DummyCost Class for ticket  #490
Adding comment to issue #490
adding a line issue #490
getting rid of the try except loop to have the train.main_loop() inform the user
of the underlying error (if there is a bug or an error there)
Merge pull request #796 from SinaHonari/n490

fixes the issue #490 updating SparseDataset to works with data_specs