Test cases for data input-output compatibility

[Neeratyoy]

Need to create unit tests to check all combination of input-output pairs of data type conversions allowed in openml-python.

For example:
ARFF -> DENSE
ARFF -> SciPy Sparse matirx
Sparse ARFF -> SciPy Sparse matrix
Sparse ARFF -> Pandas Sparse Dataframe
...