Changed DoubleMatrixDataset, it no longer extends DoubleMatrix2D

Removed the extention of DoubleMatrix2D. To be sure no DoubleMatrix2D
methods are useable on the DoubleMatrixDataset and no data is stored in
duplicate.
Merge pull request #151 from Bonder-MJ/master

Changed DoubleMatrixDataset, it no longer extends DoubleMatrix2D