WIP: Better preconditionners 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

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

TODO:
– make sure we have a good default for options_.simul.iter_tol. The gmres and bicgstab
  functions default to 1e-6. The run_block_bytecode_tests needs 1e-7. And
  GIMF/GMMET need 1e-12 in the block-decomposed version (partly because tolf=1e-8
  there)
– add tests for the three preconditioners (and ideally their suboptions)
– documentation