Implement reinit

[devmotion]

This PR is based on #49 and implements reinit! for DDEIntegrator.

[coveralls]

Coverage increased (+0.4%) to 82.528% when pulling 63bde02 on devmotion:reinit2 into 67bd419 on JuliaDiffEq:master.

[codecov]

Codecov Report

Merging #53 into master will increase coverage by 0.39%.
The diff coverage is 74.11%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   82.12%   82.52%   +0.39%     
==========================================
  Files          11       11              
  Lines         498      538      +40     
==========================================
+ Hits          409      444      +35     
- Misses         89       94       +5

Impacted Files	Coverage Δ
src/solve.jl	81.88% <64.28%> (+1.12%)	⬆️
src/integrator_interface.jl	67.2% <83.72%> (+8.66%)	⬆️
src/callbacks.jl	89.18% <0%> (-1.36%)	⬇️
src/discontinuity_type.jl	52.94% <0%> (+5.88%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 67bd419...63bde02. Read the comment docs.

[ChrisRackauckas]

Thanks for finishing this up. Looks great.