Backport changes from Houdini trunk

Only notable changes to two nodes:

- SOP_OpenVDB_Advect_Points.C
    - Add support for traillen attribute

- SOP_OpenVDB_Convert.C
    - Disable adding to seamPointGroup if we don't have pointFlags() to avoid
      out of bounds access leading to a crash. (bug #62017)
    - Use a half width of 3 voxels instead of 1 when converting fog VDBs to SDF.
    - Fix bug where converting fog volumes to SDF in Convert VDB ignored the
      isovalue. To ensure backwards compatibility, a new fogisovalue parameter
      is added that defaults to the old 0.5 value. (bug #61477)