add test and mod doc

tests/dataset_tests/README.md

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+# About dataset tests
+This tests is for testing the prepared dataset from Yahoo

tests/dataset_tests/test_dataset.py

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+
+import qlib
+from qlib.data import D
+from qlib.config import REG_CN
+import unittest
+import numpy as np
+
+
+class TestDataset(unittest.TestCase):
+
+    def setUp(self):
+        provider_uri = "~/.qlib/qlib_data/cn_data"  # target_dir
+        qlib.init(provider_uri=provider_uri, region=REG_CN)
+
+    def testCSI300(self):
+        close_p = D.features(D.instruments('csi300'), ['$close'])
+        size = close_p.groupby('datetime').size()
+        cnt = close_p.groupby('datetime').count()
+
+        print(size.describe(percentiles=np.arange(0.1, 0.9, 0.1)))
+        print(cnt.describe(percentiles=np.arange(0.1, 0.9, 0.1)))
+        # TODO: assert
+
+    def testClose(self):
+        close_p = D.features(D.instruments('csi300'), ['Ref($close, 1)/$close - 1'])
+        print(close_p.describe(percentiles=np.arange(0.1, 0.9, 0.1)))
+        # TODO: assert
+
+
+if __name__ == '__main__':
+    unittest.main()
+