Open Boundary Poisson Solver

[WeiqunZhang]

This adds an open boundary Poisson solver based on the James's algorithm. To use it, the user builds an amrex:OpenBCSolver object, which can be reused until the grids change, and then call OpenBCSolver::solve.

Currently, this is for 3D cell-centered data only. The solver works on CPU, Nvidia GPUS, and AMD GPUs. The SYCL version of a couple of kernels for Intel GPUs are to be implemented.

Checklist

The proposed changes:

• [ ] fix a bug or incorrect behavior in AMReX
• [x] add new capabilities to AMReX
• [ ] changes answers in the test suite to more than roundoff level
• [ ] are likely to significantly affect the results of downstream AMReX users
• [ ] include documentation in the code and/or rst files, if appropriate