requires_ast decorator for SCTs, as in protowhat

[machow]

with the addition of check_file, pythonwhat can create a state for a file that is not python code. In this case, the ast tree (state.student_tree) will be None. if someone tries to use check_node etc.. it will fail the SCT.

However, we either want to...

• skip these SCTs when the ast is None (as in protowhat). This would allow for custom ast parsers depending on file type, and for them to fail gracefully
• to throw an error for developers, saying they can't use these SCTs without an AST.