Implement validation for Delay and Duration groups

[happy5214]

Support for validating Delay and Duration tag groups is required as of HED schema version 8.2.0. Currently, these tag groups are only validated using the topLevelTagGroup schema attribute on the respective tags. The following validation also needs to be performed:

• Delay and Duration behave similarly to Onset and Inset syntactically, in that tag groups with that tag must contain the tag and a tag group, and nothing else (with one exception).
• Delay and Duration may appear together in the same tag group (at the first level of the group).

See #140 for further details.