fixing tracing logic and adding inhomogenous, nonlinear wave eq test for staggered grid

[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

Fixes #352. Attempted to make the code a bit more robust to possibility of shuffling variables.

Test solves the inhomogenous gas flow equations:
\rho_t + \phi_x = 0
\phi_t + a^2 \rho_x = -\beta |\phi|\phi/\rho
(Instabilities are created at long times as shown in figure, but I think this should be related to #351 and so I will not attempt to solve here.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (172cdc4) 84.99% compared to head (f606486) 62.16%.

Files	Patch %	Lines
src/discretization/staggered_discretize.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #353       +/-   ##
===========================================
- Coverage   84.99%   62.16%   -22.84%     
===========================================
  Files          41       41               
  Lines        1959     1953        -6     
===========================================
- Hits         1665     1214      -451     
- Misses        294      739      +445     

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