Ints promoted to float with many terms in sum

[baggepinnen]

The first expression below maintains integer coefficients, while adding one more term in the sum promotes all of them to floats.

julia>     -216*Tsettle^2*ogain^3 - 1296*Tsettle^2*ogain^2
-1296(Tsettle^2)*(ogain^2) - 216(Tsettle^2)*(ogain^3)

julia>     -216*Tsettle^2*ogain^3 - 1296*Tsettle^2*ogain^2 - 648*Tsettle^2*ogain
-648.0(Tsettle^2)*ogain - 1296.0(Tsettle^2)*(ogain^2) - 216.0(Tsettle^2)*(ogain^3)

This is bad since solve_multivar only likes ints and rationals (rationals also promote like the ints above)