This function returns the minimum power of F, the first argument, in
the ideal I, the second argument.
If F is not in the radical I then -1 is returned.
Example
/**/ Use QQ[x,y,z];
/**/ I := ideal(x^6*y^4, z);
/**/ IsInRadical(x*y, I);
true
/**/ MinPowerInIdeal(x*y, I);
6