title: How to downsample grid in OpenVDB
body: <p>Is there any good way to downsample voxels grid in OpenVDB?</p>
  <p>For example I have grid 8x8x8 with voxel size - 1.0, and I want to get grid 4x4x4 with voxel size - 2.0:
  <code>each voxel of new grid is some interpolation of original voxels, e. g. [0,0,0] voxel of new grid is average value of [0,0,0]-[1,1,1] (8 voxels) of original grid)</code></p>
  <p><em>P.S. There is way to do it manually but I bet openvdb has its own implementation</em></p>