Fixed NSI sequence export when Volume is being modified from other nodes.
When exporting a sequence of NSI files which contain vdb files (file SOP) that are being modified by other nodes (transform, deformers etc.), all exported NSI files were pointing to one VDB file only, causing wrong result. When the volume gets modified, we can't use the source vdb directly but instead we need to export modified result (in a new vdb file), and assign this new one to our vdbfilename attribute. If we have a sequence of NSI's files then for each frame the corresponing vdb will get exported. This fixes #389 (closed)