Change generator::set_axiom to not require setting before preprocessing

include/fractal.hpp

@@ -237,9 +237,6 @@ public:
 	 * The "axiom" is used to generate the very first generation of leafs.
 	 * Only one subgroup from the weighted group is selected.
 	 *
-	 * @pre preprocess() has not been called on this generator.
-	 *
-	 * @exception mtl::system_error If the generator has already been preprocessed.
 	 * @exception mtl::invalid If an invalid weighted group ID was suppied.
 	 */
 	void set_axiom(weighted_group_id_t wgroup);