Decide behavior with missing units on tags

[IanCa]

We need to determine the validation(and potentially downstream) behavior when a tag is missing units.

Right now we issue a warning for missing unit, even if there is a default.

We also need to determine what happens when there is no unit and there isn't a default unit specified
What if there isn't a default unit specified, but there is only one unit(such as tempatureUnits right now)

This is also posted here:
hed-standard/hed-python#792