A set of power products is factor closed iff it contains every factor of
every one of its elements. This function checks whether the given set
is factor closed (also known as "order-ideal"). It is an error if L is empty.
Example
/**/ use P ::= QQ[x,y,z];
/**/ IsFactorClosed([1, x, x^2]);
true
/**/ IsFactorClosed([one(P), y^2]);
false