Define a "region or area" of interest

Based on the existing XMP standard (supported by our apps) at http://www.exiv2.org/tags-xmp-mwg-rs.html

We need the basic region itself, which is the area that it refers to and then what it refers to (probably an XDM schema name, but potentially hierarchical)

then the area that it refers it
`['http://ns.adobe.com/xmp/sType/Area#']`

Field Name Value Type Description
`stArea:x` Short X coordinate of the area.
If `stArea:w` and `stArea:h` are specified, the area is a rectangle
centered at (`stArea:x`, `stArea:y`).
If `stArea:d` is specified, the area is a circle centered at
(`stArea:x`, `stArea:y`).
If `stArea:w`, `stArea:h`, `stArea:d` are not specified, the area is a
single point centered at (`stArea:x`, `stArea:y`).
`stArea:y` Short Y coordinate of the area.
If `stArea:w` and `stArea:h` are specified, the area is a rectangle
centered at (`stArea:x`, `stArea:y`).
If `stArea:d` is specified, the area is a circle centered at
(`stArea:x`, `stArea:y`).
If `stArea:w`, `stArea:h`, `stArea:d` are not specified, the area is a
single point centered at (`stArea:x`,`stArea:y`).
`stArea:w` Short If specified, indicates the area is a rectangle centered at
(`stArea:x`, `stArea:y`), with a width of `stArea:w` and a height of
`stArea:h`.
`stArea:h` Short If specified, indicates the area is a rectangle centered at
(`stArea:x`, `stArea:y`), with a width of `stArea:w` and a height of
`stArea:h`.
`stArea:d` Short If specified, indicates the area is a circle centered at (`stArea:x`,
`stArea:y`), with a diameter of `stArea:d`.
`stArea:unit` Open Choice Units. For example: inch, mm, pixel, pica, point.