Parallel RBF and Built-in CG solver

[Wendi-L]

What has been done:

Make RBF sampler work in both serial and parallel mode.

• Added functions to facilitate ghost cell collection and ghost cell matrix construction.

  Removes the Eigen library dependency.

• Established MUI built-in linear algebra namespace, i.e. mui::linalg, with sparse matrix system and conjugate gradient solver.
• The conjugate gradient solver is based on Algorithm 1 of the following paper: Igor Baratta, Chris Richardson, and Garth Wells. "Performance analysis of matrix-free conjugate gradient kernels using SYCL." International Workshop on OpenCL. 2022.
• In addition to the implementation of the conjugate gradient solver based on the paper above, three preconditioners (Incomplete LU preconditioner, Incomplete Cholesky Preconditioner, Symmetric Successive Over-Relaxation Preconditioner) have also been implemented in the mui::linalg namespace. The conjugate gradient solver can be worked in both w/o preconditioner mode and with one of the preconditioner modes.
• Add unit test code (a separate folder named “test”) to verify arithmetic operations of the sparse matrix, verify matrix file I/O and verify different combinations of preconditioner and CG by comparing calculated results with reference results.

  Verifications:

• The RBF sampler is verified by MUI demo 7-pseudo-diffusion. Both conservative mode and consistent mode have been tested.
• The arithmetic operations of the mui::linalg sparse matrix system have been verified by the matrix.cpp code in the “test” folder. More specifically: matrix addition, matrix subtraction, matrix multiplication, matrix transpose and matrix I/O (includes print matrix to the console and matrix file I/O in CSV format).
• The mui::linalg Conjugate Gradient solver has been verified with reference results by the conjugate_gradient.cpp code in the “test” folder. The following modes have been verified: TEST_0. CG Solver w/o Preconditioner & w/o Initial matrix Guess; TEST_1. CG solver with ILU Preconditioner & w/o Initial matrix Guess; TEST_2: CG solver with SSOR Preconditioner & w/o Initial matrix Guess; TEST_3: CG solver with IC Preconditioner & with Initial matrix Guess.

  Future works:

• Generalise ghost cell functions, so that to enable other samplers/algorithms to access and use it.
• Since the RBF sampler is no longer special compared with other samplers, some pieces of code like below can be removed.

If define RBF:
    Do this;
else:
    Do that;

[Wendi-L]

Sure, I'll withdraw this pull request.