MPX - MuseumPlusXML, an XML format that represents data from MuseumPlus

I release it as a separate project on github, to have a specific issue cue etc.

The schema should be accessible via:
 https://github.com/mokko/MPX/raw/master/latest/mpx.xsd

now.

This should always be the latest version. 

There should be different links for different version, the main directory.