Refactor dt handling

[devmotion]

Move dt handling to separate function, as suggested in SciML/DelayDiffEq.jl#88 (comment).

[YingboMa]

Thanks! LGTM

[ChrisRackauckas]

Mwtp

[coveralls]

Coverage decreased (-0.001%) to 32.042% when pulling 5299cb8 on handle_dt into c20a103 on master.

[codecov]

Codecov Report

Merging #553 into master will decrease coverage by <.01%.
The diff coverage is 30%.

@@            Coverage Diff            @@
##           master    #553      +/-   ##
=========================================
- Coverage   33.51%   33.5%   -0.01%     
=========================================
  Files          81      81              
  Lines       23613   23614       +1     
=========================================
- Hits         7914    7912       -2     
- Misses      15699   15702       +3

Impacted Files	Coverage Δ
src/solve.jl	46.26% <30%> (-1.16%)	⬇️

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