Select which solver to use by default according to user parameters

[abelsiqueira]

When the user doesn't know the solver but wants to have an intermediate level of control, e.g., fminunc(f) could mean use Nelder-Mead or use LBFGS/Newton with automatic differentiation.
We can try to set this with Value types

# For instance
function fminunc(f, ::Val{:no_derivative})
  fminunc(f, solver=:nelder_mead)
end

function fminunc(f, ::Val{:first_derivative})
  fminunc(f, solver=:lbfgs)
end

fminunc(f, s::Symbol) = fminunc(f, Val(s))

cf. @RenanOD