Composite solutions of DDE problems

[devmotion]

I started looking into SciML/DelayDiffEq.jl#64 and noticed that currently it is not possible to build ODECompositeSolutions from DDEProblems, i.e. without this PR https://github.com/JuliaDiffEq/DelayDiffEq.jl/blob/master/src/solve.jl#L59-L62 creates just a ODESolution.

[ChrisRackauckas]

merge when tests pass

[coveralls]

Coverage decreased (-0.003%) to 79.636% when pulling 834949b on composite_dde into ef4ca42 on master.

[codecov]

Codecov Report

Merging #310 into master will decrease coverage by 11.22%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           master     #310       +/-   ##
===========================================
- Coverage   79.63%   68.41%   -11.23%     
===========================================
  Files          69       69               
  Lines       23044    23028       -16     
===========================================
- Hits        18352    15754     -2598     
- Misses       4692     7274     +2582

Impacted Files	Coverage Δ
src/composite_solution.jl	0% <0%> (-71.43%)	⬇️
src/perform_step/threaded_rk_perform_step.jl	0% <0%> (-100%)	⬇️
src/composite_algs.jl	0% <0%> (-100%)	⬇️
src/dense/high_order_rk_addsteps.jl	2.12% <0%> (-97.88%)	⬇️
src/perform_step/composite_perform_step.jl	4.16% <0%> (-87.5%)	⬇️
src/callbacks.jl	0% <0%> (-86.96%)	⬇️
src/dense/low_order_rk_addsteps.jl	14.11% <0%> (-85.29%)	⬇️
src/dense/stiff_addsteps.jl	4.25% <0%> (-83.52%)	⬇️
src/caches/feagin_caches.jl	23.68% <0%> (-76.32%)	⬇️
src/perform_step/feagin_rk_perform_step.jl	25% <0%> (-75%)	⬇️
... and 29 more

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