Math JS error for FPCore input.

[zaneenders]

Getting an error for the following FPCore.

(FPCore (a b angle x-scale y-scale)
  (let* ([θ (* (/ angle 180) PI)]
         [F (* (* b a) (* b (- a)))]
         [A (/ (/ (+ (pow (* a (sin θ)) 2) (pow (* b (cos θ)) 2))
                x-scale) x-scale)]
         [B (/ (/ (* (* (* 2 (- (pow b 2) (pow a 2))) (sin θ)) (cos θ))
                x-scale) y-scale)]
         [C (/ (/ (+ (pow (* a (cos θ)) 2) (pow (* b (sin θ)) 2))
                y-scale) y-scale)])
        (- (* B B) (* (* 4 A) C))))

[elmisback]

So this is being handled in a weird way where it's casting the FPCore to MathJS and then back to FPCore to get the variable names. We should just save the FPCore and stick to using that if it's available. In general, we should shift things toward using FPCore as the primary internal representation for Odyssey.