Improve validate filename

We have used a rather inelegant way to check if something is a file when reading in a PSD:

        if file is not None and '/' not in file:
            file = os.path.join(os.path.dirname(__file__), 'noise_curves', file)
        return file

This should be replaced by something like

        if file is not None and os.path.isfile(file):
            file = os.path.join(os.path.dirname(__file__), 'noise_curves', file)
        return file

This needs to be implemented and checked that it works. We should also look for any other instances which do something similar.