Add checks for a few errors. If a check fails, then an exception would be raised.

• Check if an unknown resource was declared in a XML file.
• Check if a resource was declared outside of a File node.
• Check if a File was declared inside a File node.
• Check if a resource has childs, and throw an exception if that resource shouldn't.
• Check for repeated names, outnames and offsets.
• Check for valid variable name for each resource.

This PR is related to issue #18