Fix handling of small timesteps

Sundials solver is allowed to warn and continue if it returns a timestep
with no change in t.
If the solver doesn't recover, it will fail with eg SciMLBase.ReturnCode.MaxIters

Reverts PR #416, fixes
#420

For CVode and ARKode, it looks like this is intended behaviour: the
Sundial solver emits a warning message, with an API call
    CVodeSetMaxHnilWarns
    ARKStepSetMaxHnilWarns
to set the maximum number of warning messages printed, which is exposed
to Julia as
    max_hnil_warns
see Sundials driver code https://github.com/LLNL/sundials/blob/e8a3e67e3883bc316c48bc534ee08319a5e8c620/src/cvode/cvode.c#L1339-L1347

For IDA, there is no API like this so it's not so clear what should happen,
however the behaviour prior to #416 (restored by this PR)
for the f_noconverge test added in test/common_interface/ida.jl is to return
  SciMLBase.ReturnCode.MaxIters
which seems reasonable?

(NB: @oscardssmith I can't reproduce the issue implied by the title of
#416 so perhaps what is missing here
is the original failure case that demonstrates the issue?
The call to solve in the f_noconverge test test/common_interface/ida.jl, returns SciMLBase.ReturnCode.MaxIters,
it doesn't return with no error?

Also SciML/SciMLBase.jl#458 shows IDA printing
  [IDAS ERROR]  IDASolve
    At t = 0 and h = 1.49012e-18, the error test failed repeatedly or with |h| = hmin.
which suggests that IDA did flag an error in that case ?
)

Author: sjdaines

src/common_interface/solve.jl

@@ -1406,9 +1406,7 @@ function DiffEqBase.solve!(integrator::AbstractSundialsIntegrator; early_free =
             integrator.userfun.p = integrator.p
             solver_step(integrator, tstop)
             integrator.t = first(integrator.tout)
-            if integrator.t == integrator.tprev
-                integrator.flag = -3
-            end
+            # NB: CVode, ARKode may warn and then recover if integrator.t == integrator.tprev so don't flag this as an error 
             integrator.flag < 0 && break
             handle_callbacks!(integrator) # this also updates the interpolation
             integrator.flag < 0 && break

test/common_interface/ida.jl

@@ -117,4 +117,4 @@ isapprox(only(sol.u[end]), exp(1), rtol = 1e-3)
 f_noconverge(out, du, u, p, t) = out .= [du[1] + u[1] / (t - 1)]
 prob = DAEProblem(f_noconverge, [1.0], [1.0], (0, 2); differential_vars = [true])
 sol = solve(prob, IDA())
-@test !(sol.retcode in (ReturnCode.Success, ReturnCode.MaxIters))
+@test !(sol.retcode in (ReturnCode.Success, ))