ECT: CheckAreaLedger

[aarontran]

When using embedded boundary with enlarged-cell technique (ECT) solver, check that global face area is conserved (lent area = borrowed area) to within an automatically-determined round-off error tolerance. Also check that S_mod > 0 for all participating faces to guard against an edge case wherein multiple unstable cells can "overdraft" a neighbor cell's available area.

This PR was created to help test PR #7104, which fixes area-conservation bugs related to cross-box synchronization of lent/borrowed face area.

During review of PR #7104, it was also noticed that S_mod < 0 on a face may occur due to at least one pathological case of an under-resolved embedded boundary with cut-cells at AMReX box corners. The present PR simply aborts and does not attempt to re-work the ECT algorithm for a relatively obscure edge case.

Questions and notes for reviewers:

• Log with verbose >=1 or > 1 ?
• @ax3l 's comment on issue ECT: Init on GPU #2257 suggests that ECT boundary face extensions may be recomputed dynamically as a simulation evolves, either PEC boundary moving or AMReX regridding. Do you think this is a serious practical concern? If so, this PR may need to be edited to prevent check from running too often: maybe 1x at init, at some regular interval, only if user requests auditing, etc...
• Merge this PR after ECT: owner-unique cross-box borrowing and seam-synchronized conformal B push #7104 (CI compiles will break otherwise)

Example logging if checks passed for a 2D simulation:

                                   ___
    __        __             ___  /  /
    \ \      / /_ _ _ __ _ __\  \/  /
     \ \ /\ / / _` | '__| '_ \\    /
      \ V  V / (_| | |  | |_) /    \
       \_/\_/ \__,_|_|  | .__/__/\  \
                        |_|       \__\

Level 0: dt = 1.179327168e-10 ; dx = 0.05 ; dz = 0.05
--- INFO    : Embedded Boundary: ECT CheckAreaLedger after 1-way pass (idim=1)
             imbalance sum(S-S_mod) = 0.000000e+00 tolerance = 5.107026e-15 OK.
             Modified faces min(S_mod) = 1.000000e-03 > 0 OK.
--- INFO    : Embedded Boundary: ECT CheckAreaLedger after 8-way pass (idim=1)
             imbalance sum(S-S_mod) = -2.249720e-18 tolerance = 5.129230e-15
             OK. Modified faces min(S_mod) = 2.500000e-04 > 0 OK.

Grids Summary:
  Level 0   16 grids  1024 cells  100 % of domain
            smallest grid: 8 x 8  biggest grid: 8 x 8

Example logging if a check fails:

0::Assertion `balance && no_overdraft' failed, file "[...]/warpx/Source/EmbeddedBoundary/WarpXFaceExtensions.cpp", line 203, Msg:
### ERROR   : Embedded Boundary: ECT CheckAreaLedger after 1-way pass (idim=1)
#            imbalance sum(S-S_mod) = -1.734723e-18 tolerance = 9.436896e-16
#            OK. Modified faces min(S_mod) = -5.000000e-04 but expected > 0 for
#            all faces, exiting! Try resolving embedded boundary with more
#            cells, varying AMReX grid layout, and/or filing bug report.