Geometry Nodes: Add volume grid creation nodes

Add three nodes to the currently experimental volume grid nodes:
- **Mesh to Density Grid**
- **Mesh to SDF Grid**
- **Points to SDF Grid**

These nodes are just wrappers over basic openVDB utilities.
The difference from existing volume nodes is that they output
a grid directly instead of a geometry set.

@@ -25,0 +30,4 @@
+ public:
+  OpenVDBParticleList(const Span<float3> positions,
+                      const Span<float> radii,
+                      const float voxel_size)

@@ -0,0 +22,4 @@
+      .min(0.01f)
+      .max(FLT_MAX)
+      .subtype(PROP_DISTANCE);
+  b.add_input<decl::Float>("Interior Band Width")

@@ -204,0 +211,4 @@
+  bke::VolumeTreeAccessToken access_token;
+  return mesh_grid ?
+             BKE_volume_grid_add_vdb(
+                 *volume, name, mesh_grid.get_for_write().grid_ptr_for_write(access_token)) :

@@ -91,4 +111,0 @@
-{
-  openvdb::FloatGrid::Ptr new_grid = points_to_sdf_grid(positions, radii);
-  new_grid->transform().postScale(voxel_size);
-  new_grid->setGridClass(openvdb::GRID_LEVEL_SET);

@@ -20,2 +21,3 @@
-struct OpenVDBParticleList {
+class OpenVDBParticleList {
+ public:
   using PosType = openvdb::Vec3R;