Proposal: Make iteration based attributes "required" only if iteration data is stored

[DavidSagan]

The following iteration related attributes are marked as required:

• iterationEncoding
• iterationFormat
• time
• dt

This makes no sense unless iteration based data is being stored and therefore these attributes should only be required when storing iteration based data.

[ax3l]

Thank you for the proposal. I agree that this can simplify the standard and will incorporate it with the upcoming changes I need to do for iterationEncoding for ADIOS2.