Deserialization from the standard XML encoding

[MarcoCostantini]

The Json encoding is cool, but the OpenMath standard requires the XML encoding, and now almost all the other implementations of OpenMath support only the XML encoding; without it, it is not possible for the other implementations to interoperate with openmath-js.

Please implement also the deserialization from the standard XML encoding

[MiguelMJ]

Any news on this?

[nathancarter]

Hello. My current projects no longer use this library, so I do not have time myself to add XML-reading functionality. I would gladly accept a PR. Sorry that's not as helpful as you might have hoped!