changed cutoffs slightly
Added general rref method and moved old rref to rref_fflu
Added general is_in_hnf, is_in_rref and is_in_snf functions to avoid code duplication and for general use
Test that the hnf_transform function outputs a hnf matrix, this is currently tested implicitly in t-hnf, but in the future, who knows?
Changed nullspace to use general rref function again