The three principle aircraft axes are not an unambiguous specification of orientation, because it depends on the order in which they are applied. https://en.wikipedia.org/wiki/Aircraft_principal_axes

For instance, roll by 180, then pitch by 90, would nose-dive the plane, whereas pitch by 90 then roll by 180 would point the plane straight up and then roll it (while still going up).

Two possibilities:

1. (easier for now) Explicitly specify order that angles are applied. I suggest yaw, then pitch, then roll.

2. Use "quaternions": two vectors forward and normal, where forward specifies equivalent information to pitch and yaw, by saying which direction the forward direction of the Helix is pointing, and normal is a vector perpendicular to forward indicating the roll along the forward axis.

See also UC-Davis-molecular-computing/scadnano#546.

There's two issues here:

1. What we store internally and in the JSON. I think that should eventually be the forward and normal vectors, which makes this more straightforward for oxDNA export, for example.

2. What interface is provided in the web interface and Python API. Some people might find it easier to think in terms of aircraft principle axes, so we might want to keep that as a way to specify the orientation. For instance, designs that set pitch to rotate in the main view are more straightforward to just give an angle (like 45 degrees), than to think about the axes and what the forward vector should be. (which for 45 degree pitch I think would be the vector (0,1,1))

This should be implemented before #139, to make it simpler to do that export, since oxDNA supposedly uses the quarternion approach.