added nullity in _howell and generic top-level function
Farey lift added for n_Z (rintegers). Not optimal (nor completely
correct in the border cases), but functional
solveAx_dixon and friends: Farey-lift over Z (no error checking) and
start of generic dispatcher. Problem(s)
 - under determined systems can have solutions in Z that disxon will not
   find
 - large systems will run out of memory in howell due to the non-modular
   HNF
 - dixon (as of now) will not find the kernel