Remove arbitrary leaf extensions #5
Description
In previous versions of the HED schema, extensions were allowed from any leaf node, regardless of whether or not there is an extensionAllowed attribute on that node's path. Now, this is being phased out in favor of requiring this attribute somewhere in a node's path in order for the extension to be legal. In particular, the leaf extension code should be removed from TagValidator, including the _leaf_extensions field and the is_leaf_extension_allowed_tag method. The leafExtension error type should also be removed.
There is a branch with these changes.