Staggered interpolations

[cmhyett]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Working to fix #351. Need to utilize variable staggering for non-derivative terms. I currently implement this as a simple average between neighboring points, but in general we should leverage MoL's already built-in interpolations.

Attached are generated code before/after for gas-flow equations.
interpolation_results.txt

CI will break on this commit due to lack of support for periodic bcs, but I'll resolve it in the next.

[codecov]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (172cdc4) 84.99% compared to head (f504463) 61.78%.

Files	Patch %	Lines
src/discretization/discretize_vars.jl	0.00%	12 Missing ⚠️
src/discretization/staggered_discretize.jl	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #354       +/-   ##
===========================================
- Coverage   84.99%   61.78%   -23.22%     
===========================================
  Files          41       41               
  Lines        1959     1965        +6     
===========================================
- Hits         1665     1214      -451     
- Misses        294      751      +457     

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[xtalax]

Looks good, could use higher order interpolations though, see the nonlinear laplacian for how