NXxas refactoring

[woutdenolf]

WIP

Rendering: https://hdf5.gitlab-pages.esrf.fr/nexus/nxxas/classes/applications/NXxas_new.html

NXDL questions:

• how to define list of enum as values
• when enum field X has a specific value, make field Y required OR how to define enum properties
• possibly link to
• how to make a field required in a base class

[woutdenolf]

Replaced by https://github.com/nexusformat/definitions/pull/1352