Remove old DataSet implementation. Create new DataSets. (#151)

* Remove old DataSet implementation.

The new Dataset will be used as data source for packages. All DataSets will be capable of sampling to DataFrame, so feature exploration can be done with other libraries.

* Raise error when sample is larger than number of rows.

Author: qimingj

datalab/mlalpha/__init__.py

@@ -22,7 +22,7 @@
 from ._job import Jobs
 from ._summary import Summary
 from ._tensorboard import TensorBoardManager
-from ._dataset import DataSet
+from ._dataset import CsvDataSet, BigQueryDataSet
 from ._package import Packager
 from ._cloud_models import CloudModels, CloudModelVersions
 from ._confusion_matrix import ConfusionMatrix