Federated Read Sparse Optimization

Optimize the handling of sparse input in the method readBlobFromFederated in src/main/java/org/apache/sysds/runtime/controlprogram/caching/MatrixObject.java.
This task has a "TODO" comment in the code.

The task is to exploit meta data of the number of non-zeros (if existing) and allocate the resulting local matrix in sparse instead of dense representation before processing the federated responses.