Boundary Operation for most operators like sample

I found it is hard to use openvdb in fluid simulation because I can't use some operators like 'sample' 'gradient' which don't care about boundary condition. Now I have to set value in the Valueoff voxel, but it is not a very well solution in multithread environments.

Is there any possible to add boundary ops like PoissonSolver in the future or there are another better treatment of boundary condition? THX

Review the existing sample and gradient operators alongside PoissonSolver, focusing on how boundary conditions and Valueoff voxels are handled. Define the desired boundary-operation behavior for fluid simulation and determine how it must remain safe in multithreaded use.

Written by the indexing model from the issue text.