Orthogonalize the Galactic-to-Ecliptic rotation matrix to machine precision to avoid loss of precision when using the transpose for the inverse.
Only store the Layout special attributes in the packed object. Mirror the non-special Layout attributes in the packed object.
In Layout, make 'removed' a property (computed from the index).
In Layout, always set the index to None if it doesn't change the ordering and if it doesn't indicate that a component has been removed.
Implement Layout selection.
Implement Instrument.__len__ to return the number of non-removed elements, Instrument.__getitem__ to copy the Instrument for the selected non-removed detectors and Instrument.__iter__ to iterate over the non-removed detectors.