WIP: Better preconditionner for stack_solve_algo={2,3} (GMRES + BiCGStab solvers)

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 (1e-12 seems needed for GIMF in the block-decomposed version)
– bytecode
  + improve implementation of ILU preconditioner by using the structure in
    options_
  + forbid other preconditionners
– handle the error flag of gmres and bicgstab?
– remove the persistent variables in sim1.m
– turn various settings as user-accessible options

[skip ci]