Make ArcLineSet less inefficient

The properties such as wavelength are a major performance bottleneck. Please add caching or otherwise make this more efficient.

def wavelength(self):
        """Array of reference wavelength, nm (`numpy.ndarray` of `float`)"""
        return np.array([ll.wavelength for ll in self.lines], dtype=float)

Once you know that this how the code was written it's not too bad, but innocent-looking code can be very slow.