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

TODO:
– better understand how the tolerance of GMRES and BiCGStab influence the
  result (hardcoded to 1e-12, needed for GIMF in the block-decomposed version);
  also put that value in options_ and maybe give it an interface?
– implement ITERSTACK for the block-decomposed version
– do the same in bytecode
– leave ILU as an alternative preconditionner?
– handle the error flag of gmres and bicgstab?
– remove the persistent variables in sim1.m
– turn various settings as user-accessible options

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