Add some DDE problems from DelayDiffEq.jl

[devmotion]

Hi!

I discovered that the analytical solution to one of the test problems in DelayDiffEq.jl is incorrect. The errors in https://github.com/JuliaDiffEq/DelayDiffEq.jl/blob/master/test/constrained.jl#L32 and https://github.com/JuliaDiffEq/DelayDiffEq.jl/blob/master/test/constrained.jl#L81 do not affect the calculations since they occur outside of the time interval of the corresponding DDE problems. Nevertheless, I thought it could be beneficial to add the most used test problems in DelayDiffEq.jl to this repository, in order to remove redundancy and possible additional errors.

I recalculated both analytical solutions in https://github.com/JuliaDiffEq/DelayDiffEq.jl/blob/master/test/constrained.jl with Mathematica.

[ChrisRackauckas]

Nice. I've been meaning to get around to this. Thanks for catching that error as well!

[coveralls]

Coverage remained the same at 0.0% when pulling 1c8f176 on devmotion:dde_problems into ae76320 on JuliaDiffEq:master.

[coveralls]

Coverage remained the same at 0.0% when pulling 1c8f176 on devmotion:dde_problems into ae76320 on JuliaDiffEq:master.

[codecov-io]

Codecov Report

Merging #9 into master will not change coverage.
The diff coverage is 0%.

@@         Coverage Diff          @@
##           master    #9   +/-   ##
====================================
  Coverage       0%    0%           
====================================
  Files           5     6    +1     
  Lines         142   210   +68     
====================================
- Misses        142   210   +68

Impacted Files	Coverage Δ
src/dde_premade_problems.jl	0% <0%> (ø)

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