HSS Approximation of Dense Matrices

The HSS code can be found in the src/HSS/ subdirectory. All HSS code is in a namespace called HSS. The class for a sequential/multithreaded HSS matrix is HSSMatrix<scalar>, while the distributed memory HSS class is HSSMatrixMPI<scalar>. For examples of the usage of these classes, see the test code in test/test_HSS_seq.cpp and test/test_HSS_mpi.cpp respectively. There is also one sequential example in examples/MLkernel.cpp, which uses HSS compression for kernel matrices as used in certain machine learning applications, see for instance [2]. This part of the code will be better documented in future STRUMPACK releases.

__Currently the class HSSMatrixMPI<scalar> cannot be used for on a communicator with a single MPI process. In that case, one should use the sequential HSSMatrix<scalar> class.__