Refactor code that checks for field and group occurrences

[rayosborn]

• Moves code that check for valid values forminOccurs, maxOccurs, optional, and recommended to a separate function.
• Adds checks on the optional and recommended attributes to the FieldValidator.
• Outputs dimension warnings as informational messages, since their specfications in NXDL files contain a number of anomalies.
• Reduces the deprecated use of signal and axis attributes in NXfields to a warning.

[rayosborn]

@takluyver, I have refactored the code that was included in your PR #247, because your fix was also useful in fixing the FieldValidator as well as the ApplicationValidator. The new version has now been merged into the main branch. If the new code no longer fixes your original issue, please let me know. If there are no problems, I will try to release this as a bug release in the next week.

[takluyver]

Thanks! I've given it a quick try and it seems to still do what I'm expecting. 👍

Demoting the dimension-related messages to INFO also hides the issue I had in #248. I hope you can get the issues with the spec worked out, though.

[rayosborn]

Thanks for confirming. I think that the way that dimensions are defined within the NeXus standard will require a lot more thought. Many of the definitions in the base classes are far too restrictive and/or inconsistent with common usage, and fixing them to work with rigorous validation tools won't be trivial.