Implementation of CSR sparse format.

[jacobdwatters]

The current SparseMatrix class is implemented in the sparse COO format. However, with just a rudimentary CSR implementation, a significant performance improvement for sparse-sparse matrix multiplication was observed. As such, the following changes should be made.

• Rename the SparseMatrix class to SparseCooMatrix.
• Implement a SparseCsrMatrix class which stores matrices int the compressed sparse row (CSR) format.
• The mult method for all sparse matrices should return a dense matrix by default.
  • Add a mult2Sparse method for sparse matrices which computes the sparse-sparse matrix multiplication but stores as a sparse matrix.

[jacobdwatters]

The CSR format may also enable a boost in sparse concurrent matrix multiplication.

[jacobdwatters]

Update:

• The previously existing sparse matrix implementations have been renamed to include Coo in the name to indicate they are stored internally in coordinate format.
• Much of the functionality for real sparse CSR matrices has been implemented
  • Test coverage is poor. More tests need to be written.