Enum checking of attributes

[rettigl]

Currently, attributes are not being checked against possible enumerations. This functionality should be added.

Steps to reproduce:
Follow the conversion notebook here: https://nomad-lab.eu/prod/v1/gui/user/uploads/upload/id/iM3-bEttS060E2-GIqaSBA
The @vector attributes of the transformation classes should be specific strings according to NXmpes_arpes, and the numeric values passed don't match these strings (see also #555). Instead, these should be checked agains the enumeration, and a warning be raised potentially.

[rettigl]

Possible fix in #557