Add a routine to get the final residual from a linear solver

[GiudGiud]

It would be useful for some linear solves where we currently output the initial and never iterate the solve so it just looks unconverged

EDIT: on hold, probably can do without

[roystgnr]

We return the final residual from solve(), right? And then LinearImplicitSystem saves it for use in final_linear_residual(). I don't know if we have any way to get it from a nonlinear system or solver, but that's in part because it doesn't a priori make sense for a nonlinear solver, which may operate through some means other than repeated linearization.