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:
– better understand how the tolerance of GMRES and BiCGStab influence the
  result (1e-12 seems needed for GIMF in the block-decomposed version)
– handle the error flag of gmres and bicgstab? (already done in bytecode)
– turn various settings as user-accessible options
– add tests for the three preconditioners

[skip ci]