correct ordering of sys.dvs in ode_order_lowering

[simeonschaub]

This should fix #219.

[codecov]

Codecov Report

Merging #220 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   88.32%   88.32%           
=======================================
  Files          15       15           
  Lines         497      497           
=======================================
  Hits          439      439           
  Misses         58       58

Impacted Files	Coverage Δ
src/systems/diffeqs/first_order_transform.jl	100% <100%> (ø)	⬆️

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[ChrisRackauckas]

Ahh, it's a case of #214. While this isn't a good solution in general, it's reasonable enough to understand for now, at least until we get a better #214 solution. Thanks!