Use symmetric block matrix for complex coarse solve

[zhanggy2004]

Thanks for developing and maintaining this nice project.

For "complex coarse solve", instead of solving

$$A_1\begin{pmatrix}x_r \\ x_i \end{pmatrix}:=\begin{pmatrix} A_r & -A_i \\ A_i & A_r \end{pmatrix} \begin{pmatrix}x_r \\ x_i \end{pmatrix}=\begin{pmatrix}y_r \\ y_i \end{pmatrix},$$

we can solve

$$A_2\begin{pmatrix}x_r \\ -x_i \end{pmatrix}:=\begin{pmatrix} A_r & A_i \\ A_i & -A_r \end{pmatrix} \begin{pmatrix}x_r \\ -x_i \end{pmatrix}=\begin{pmatrix}y_r \\ y_i \end{pmatrix}.$$

The idea is similar to mfem::ComplexOperator::Convention::BLOCK_SYMMETRIC.

When $A_r$ and $A_i$ are both symmetric, $A_2$ will also be symmetric, which can be utitlized by MUMPS solver for more efficient solve.

I realized that $A_r$ and $A_i$ are unsymmetric when there is periodic boundary (?), but am not sure if there are other exceptions. Please let me know your suggestions for the correct/better approach. Thanks.