Lazy load of pynxtools schema in NOMAD

[lukaspie]

Loading of the pynxtools schema package can take a long time and eat up a lot of memory. We should explore options on how to limit the schema to what is required.

Just some thoughts:

• We could lazy load the schema only when it is needed in the parser
• We could only load parts of the schema depending on what each parser needs (i.e., if I use NXmpes, I only load that part of the schema that has the mpes appdefs as well as all the base classes that are inherited)
• Regarding the previous point, we must think how we can deal with that in the app.