#39 Sky plane reference frame

[ajtribick]

A reference frame for orbital elements in the sky plane would be useful, e.g. for extrasolar planets. Currently it can be achieved by

OrbitFrame {
TwoVector {
Center "star"
Primary {
Axis "z"
RelativePosition { Target "SSB" }
}
Secondary {
Axis "x"
ConstantVector { Vector [0 0 1] Frame { EquatorJ2000 { Center "star" } } }
}
}
}

But some kind of shorthand for this would be useful e.g.

OrbitFrame {
SkyPlane { Center "star" }
}