[SYSTEMDS-2760] Performance sparse-sparse transpose operations

This patch makes two minor performance improvements to multi-threaded
sparse-sparse transpose operations. The tasks get column groups of the
input and append outputs to the sparse output rows. For tall&skinny
sparse matrices there were two shortcomings.

First, we aligned the minimum column group size to 8, which is only
required for dense-dense transpose operations. This improved performance
on above scenario.

Second, if the number of cores is larger than the number of columns,
cache blocking and related position maintenance only adds unnecessary
overhead. Accordingly, we now use added a special case for these
scenarios.

On a scenario of a 2.5M x 50 matrix with sparsity = 0.1, the total
execution time of 10 transpose operations improved from 2.7s to 2.5s
(with 1) and 1.9s (with 1 and 2) respectively.
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tree: 95eff92f0118cb295ee1fce9f9351885668a7971
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  16. pom.xml
  17. README.md
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Apache SystemDS

Overview: SystemDS is a versatile system for the end-to-end data science lifecycle from data integration, cleaning, and feature engineering, over efficient, local and distributed ML model training, to deployment and serving. To this end, we aim to provide a stack of declarative languages with R-like syntax for (1) the different tasks of the data-science lifecycle, and (2) users with different expertise. These high-level scripts are compiled into hybrid execution plans of local, in-memory CPU and GPU operations, as well as distributed operations on Apache Spark. In contrast to existing systems - that either provide homogeneous tensors or 2D Datasets - and in order to serve the entire data science lifecycle, the underlying data model are DataTensors, i.e., tensors (multi-dimensional arrays) whose first dimension may have a heterogeneous and nested schema.

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