add ordering choice in ODESystem construction

[ChrisRackauckas]

No description provided.

[codecov]

Codecov Report

Merging #164 into master will decrease coverage by 0.78%.
The diff coverage is 20%.

@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
- Coverage   93.02%   92.24%   -0.79%     
==========================================
  Files          12       12              
  Lines         459      464       +5     
==========================================
+ Hits          427      428       +1     
- Misses         32       36       +4

Impacted Files	Coverage Δ
src/systems/diffeqs/diffeqsystem.jl	87.9% <20%> (-2.86%)	⬇️

---

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 9acd841...270ec2b. Read the comment docs.