Make QCX methods callable stand-alone

[MichaelClerx]

They are used as such in run_herg_qc, tested individually, and so code should be written to anticipate that

• Make each a standalone function, with any required input as args
• Turn hERGQC object into either a function, or a much simpler class. If it's going to have hardcoded numbers, it might be best to keep it as a class so that we can subclass it
• Probably want to replace all the window stuff with just calling with the view required? E.g. instead of passing in recording, window and then operating on recording[window[0], window[1]] just make the user pass that in in the first place