Should `SolverParams` remain in DFG and be serialized?

[Affie]

SolverParams is currently serialized in FileDFG

I came across this comment by @dehann in #507_:

I would say that this is a pretty strong case that should work... There is no point in storing solver data in CGDFG object. At one stage we were considering completely removing SolverParams as a separate object (not kept inside DFG), and might still happen in the future. Other Julia solvers have their option parameters separate from the problem -- so I agree with Sam: "I would think if there is a error verbose=true note to say that your SolverParams don't match, although I don't think that should stop copying the graph."

If we wan't to remove SolverParams we should do so for DFG v1, otherwise I would suggest we add it to all drivers.