Add writing of .v3draw format for easy loading with Vaa3D.

Vaa3D (http://vaa3d.org ) is an open-source program for 3D
visualization and analysis of bioimage datasets.  .v3draw is a
minimalist format designed for rapid writing and reading with Vaa3D,
currently supported by Fiji for reading and writing.  This commit
provides .v3draw writing capability for bioformats.