FkIK Switch

As per kind instructions form Brian Kenny on the Rigging 101 forum, the x-axis remains pointing down the ik line, point constrained between the fk and ik knee. The z and y translates are then zero'd out to return it along the ik line.

I've drawn a 1 degree spline to illustrate the angle I need the twist control to rotate in order to match up with the fk knee's rotation.

Many thanks to Brian for his help on this - he's probably saved me about 2 weeks work!