linear solvers: future todos

[fnrizzi]

• need to document that for iterative solvers we also have knobs for setting params

• change the solve method to return a "status" object with results on convergence, num of iterations, etc.
  for direct methods, what should this return? for iterative methods, what should this return?
  gmres works but is very complicated internally, we need to explain all internal logic for how this was done and possibly flag it as experimental

[fnrizzi]

this is a follow up on #751