Chemistry in MathML needs to be identified in metadata

[GeorgeKerscher]

On the User Experience Guide for Accessibility Metadata call, we discovered that in the EPUB Accessibility Metadata, we have no way to identify that the MathML markup is for Chemistry. ONIX has this ability.

To help, Neil writes: For chemistry, there is a "chemical-element" property and there is a "chemical-formula" property. The latter might switch the speech for "=" from "equals" to "double bond".

So, we think this functionality needs to be added. Once this is available, we can insert it to the UX Guide.