Verify top-level tag group rules in current spec

[happy5214]

During our meeting yesterday, @VisLab and I discussed the rules regarding tags marked with the topLevelTagGroup schema attribute. The result was this description of each tag's rules:

• Definition tag groups
  • ... can appear multiple times in an event line (as separate top-level groups).
  • ... must have a single inner tag group.
  • ... may be duplicated within an event file as long as they have an equivalent inner tag group.
• Onset and Offset tag groups
  • ... can appear multiple times in an event line (but not at the (as separate top-level groups) with different Def or Def-expand group name/value pairs.
    • See Synchrony issue 2: Event file rows with same onsets hed-schemas#65 (comment) for notes on repeated name/value pairs in event lines with the same time.
  • ... may have a single inner tag group (Onsets only).
• Event-context tag groups
  • ... must only contain zero or more inner tag groups (apart from Event-context itself).
• Duration and Delay tag groups
  • ... may have any inner tags or tag groups not otherwise prohibited (e.g. Onset and Offset).

As a clarification, a "name/value pair" in the JavaScript validator's parlance is a definition's name and (optional) placeholder value, i.e. the identifier for the definition.

[VisLab]

We have modified the rules for Definition so that definitions can only appear in separate keys in JSON sidecars.

As a further restriction on Definition --- certain tags such as Condition-variable and Task may only appear once in a Definition. We need an attribute for this.