returned support for sparse matrixes (which was depricated)

[iamrinni]

In current version support of sparse matrixes was unexpectedly depricated and data conversion was switched to DMatrix function only, which returns this message if we try to add sparse matrix as input data:

[https://github.com/aksnzhy/xlearn/blob/499a1836532e6efd9643bd1d17d76f5d874f8903/python-package/xlearn/data.py#L42]

As previous interface included support and conversion of sparse matrixes, it would be a good point to keep it working. Also it might be that DMatrix function itself could be performed to be able to solve this issue.

[iamrinni]

@aksnzhy @etveritas hi! any updates with it?

[orenov]

@aksnzhy @etveritas Any updates on this?