Correction to EB volume fraction for anisotropic grids (#4587)

## Summary

- This is a complementary PR to PR
#4586 by @nataraj2, for
enabling EB capabilities of AMReX for anisotropic grids.
- This PR makes changes to the implementation of volume fraction and
boundary centroid to extend them for anisotropic cases.
- These changes are backward compatible with isotropic grids, therefore,
it is safe for users of isotropic grids.

## Additional background

The use of anisotropic grids is significant in ERF applications.

## Checklist

The proposed changes:
- [x] fix a bug or incorrect behavior in AMReX
- [ ] 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

Author: skang67

Src/EB/AMReX_EB2_3D_C.cpp

@@ -109,12 +109,12 @@ void set_eb_data (const int i, const int j, const int k,
     Real aay = 0.5_rt*(aym+ayp);
     Real aaz = 0.5_rt*(azm+azp);
     Real B0 = aax + aay + aaz;
-    Real Bx = -nx*aax + ny*(aym*fcy(i,j,k,0)-ayp*fcy(i,j+1,k,0))
-                      + nz*(azm*fcz(i,j,k,0)-azp*fcz(i,j,k+1,0));
-    Real By = -ny*aay + nx*(axm*fcx(i,j,k,0)-axp*fcx(i+1,j,k,0))
-                      + nz*(azm*fcz(i,j,k,1)-azp*fcz(i,j,k+1,1));
-    Real Bz = -nz*aaz + nx*(axm*fcx(i,j,k,1)-axp*fcx(i+1,j,k,1))
-                      + ny*(aym*fcy(i,j,k,1)-ayp*fcy(i,j+1,k,1));
+    Real Bx = -nx*aax + ny*(aym*fcy(i,j,k,0)-ayp*fcy(i,j+1,k,0)) * dx[0]/dx[1]
+                      + nz*(azm*fcz(i,j,k,0)-azp*fcz(i,j,k+1,0)) * dx[0]/dx[2];
+    Real By = -ny*aay + nx*(axm*fcx(i,j,k,0)-axp*fcx(i+1,j,k,0)) * dx[1]/dx[0]
+                      + nz*(azm*fcz(i,j,k,1)-azp*fcz(i,j,k+1,1)) * dx[1]/dx[2];
+    Real Bz = -nz*aaz + nx*(axm*fcx(i,j,k,1)-axp*fcx(i+1,j,k,1)) * dx[2]/dx[0]
+                      + ny*(aym*fcy(i,j,k,1)-ayp*fcy(i,j+1,k,1)) * dx[2]/dx[1];
 
     vfrac(i,j,k) = 0.5_rt*(B0 + nx*Bx + ny*By + nz*Bz);