Purpose: describe how vap versioning is done.

The vap file itself is versioned, providing the ability for a new Autoplot to write vap files which an old version can read. For example, the "scale" property is added to the Axis node, but old versions of Autoplot won't know what this is and will fail.

Currently Autoplot writes out v1.07 files, and 1.08 exists as a marker that auto-ranging should be performed as prescribed in the file.

v1.09 introduces the scale property, but earlier Autoplot versions fail when reading these because the binding code doesn't allow for bindings to unknown properties. Autoplot v2017a_4 will correct this by dropping bindings to properties it doesn't know with a warning. Release 20171021a will handle this by writing a v1.08 file, but will remove the bindings between scale.