[BUG]Train/Maglev does not give an extension warning

[VisLab]

1. parseHedString does not give an extension warning for Train/Maglev. I am not sure where this is checked now, but I think it should be in parseHedString or a function that it calls when we finish the refactoring. I am posting this issue as a reminder. I may add to this issue with other issues that I encounter as I translate the tests.

2. validating sidecar does not detect missing definitions.

3. most detection of unit/unit class errors should be done when HED string is originally parsed. It seems to be done much later in the process.

4. The requireChild implementation seems to be problematic. The parseHedString correctly detects that Duration has requireChild. However event.spec.js is detecting that Label requires a child, which it does not.

5. Nested definitions do not seem to be detected in bidsValidation

6. parseHedString should detect the issues in Object/# and Object/Thingie/#. The Object tag cannot have a placeholder.