XML Backend

We could add an XML backend, which allows for easy verification with schema's from web development.

Somewhat similar to our JSON backend (which is more compact), XML has a more natural and verbose handling of "attributes" and "data".

This is a good way to get started when learning openPMD-api.

### Potentially Interesting Dependency for Backend

- https://github.com/zeux/pugixml
- https://github.com/leethomason/tinyxml2
- https://github.com/fffaraz/awesome-cpp#xml

Or, converting rapidxml & yaml-cpp objects to our (existing) `nlohmann:json` objects:
- https://github.com/mircodezorzi/tojson