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[bot]]

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 :warning:
src/discretization/staggered_discretize.jl	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.