adding support for custom call to inner linear solve in SNES

[Lisandro Dalcin]

I would like to add some support in SNES to enable users to customize
the inner call to KSPSolve(). The approach amount to implement
something like this:

#undef __FUNCT__
#define __FUNCT__ "SNES_KSPSolve"
PetscErrorCode SNES_KSPSolve(SNES snes, KSP ksp, Vec b, Vec x)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (snes->ksp_ewconv) { ierr =
SNESKSPEW_PreSolve(snes,ksp,b,x);CHKERRQ(ierr);  }
  if (snes->ops->linsolve) {
    ierr = (*snes->ops->linsolve)(snes, ksp, b, x);CHKERRQ(ierr);
  } else {
    ierr = KSPSolve(ksp,b,x);CHKERRQ(ierr);
  }
  if (snes->ksp_ewconv) { ierr =
SNESKSPEW_PostSolve(snes,ksp,b,x);CHKERRQ(ierr); }
  PetscFunctionReturn(0);
}

and, of course, add a routine to let users set their custom linear
solve routine.

Is this fine? Is it open-door for too-much user hackery? There could
be a better way to enable something like this?

I am also thinking (if there is no objection) in adding support for
setting a PreSolve and a PostSolve routine, but not sure what the
interface of the callback functions should be, and if the presolve
routine should be called after of before SNESSetUp().


-- 
Lisandro Dalcín
---------------
Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
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