Datetime values in rule validation is constrained to subset of ISO 8601

[servilla]

Datetime values as found in EML metadata rules are constrained to a subset of ISO 8601 because of limitations of the Python dateime fromisoformat method. This is primarily because a lack of mappings between Python datetime formats and those in ISO 8601. A better approach is to write a datetime mapper so that datetime string value may be verified in Python's datetime module.