Add the matching (augment_path) for the Pantelides algorithm

[junpenggao233]

https://epubs.siam.org/doi/abs/10.1137/0909014

I have implemented the augment path algorithm in the paper. I will open follow-up PRs to complete the entire Pantelides algorithm.

[codecov]

Codecov Report

Merging #117 into master will increase coverage by 2.37%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #117      +/-   ##
==========================================
+ Coverage   92.59%   94.97%   +2.37%     
==========================================
  Files          12       12              
  Lines         459      358     -101     
==========================================
- Hits          425      340      -85     
+ Misses         34       18      -16

Impacted Files	Coverage Δ
src/ModelingToolkit.jl	75% <ø> (ø)	⬆️
src/systems/diffeqs/pantelides.jl	100% <100%> (ø)
src/systems/nonlinear/nonlinear_system.jl	86.66% <0%> (-7.28%)	⬇️
src/variables.jl	89.18% <0%> (-5.55%)	⬇️
src/function_registration.jl	96.55% <0%> (-3.45%)	⬇️
src/systems/diffeqs/first_order_transform.jl	100% <0%> (ø)	⬆️
src/direct.jl
src/utils.jl	94.59% <0%> (+1.04%)	⬆️
... and 5 more

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[ChrisRackauckas]

What's the plan for integrating this with the MT expression trees? I don't quite see it yet.