Implement preconditioners as in UMFITER and ITERSTACK:
– For UMFITER, on first iteration, do a full LU decomposition, and use it as a
  preconditioner
  in further iterations (similar to UMFITER in TROLL).
– For ITERSTACK, build a preconditioner based on a small LU decomposition
  repeated several times across the stack

By the way, uniformize the preconditioners for solve_algo={7,8} across the
different implementations.