New sparse matrix block representation

[Matthias Boehm <mb...@us.ibm.com>]

Just a heads-up: the week after our 0.9 release, I'll make major changes to
our core matrix block library by introducing a new abstraction for sparse
matrix blocks (SYSTEMML-377). This will affect all operations of all
backends. The benefit is a more memory-efficient representation, which will
significantly improve performance over sparse matrices for CP and Spark
(less memory bandwidth requirements, less spilling, less garbage
collection). However, if you encounter performance degradations or other
issues, please file JIRAs or let me know asap in order to minimize the time
of instability. Thanks.

Regards,
Matthias